Runes are one of two options all summoners have to customize their champions before the game begins. You may buy them from the in-client store and save them to a page in your rune book, which is accessed from your profile page. You can then select a rune page at champion select to give you bonus stats in-game. Some details:
-Runes may be purchased for Influence Points only. IP is earned as a reward for each game you play; Conversely, RP (Riot Points) can only be purchased with real-world money. Riot's business model does not allow you to buy game-affecting content with RP.
-They come in three tiers of power. Summoners below Level 10 are restricted to Tier 1 runes. At higher levels, you may buy Tier 2 runes. Summoners above Level 20 may buy Tier 3 runes.
-Runes come in four types:
Marks (red) specialize in offensive stats
Seals (yellow) specialize in defensive stats
Glyphs (blue) specialize in magical stats
Quintessences (purple) are the most powerful and offer a wide range of choices
List of Runes
-You may combine runes of the same tier in your book to generate new ones. Sacrificing two runes creates one in the same tier, while using five will create a rune one tier higher. Combining runes is cheaper IP-wise than buying them outright, but the result is random. Note that the resulting rune may be of the same type as one of the ingredient runes.
-You unlock a new rune slot with every level you gain. They are unlocked in the order: mark -> seal -> glyph, with a new quint slot opened at levels 10, 20, and 30. Thus there are nine slots each for marks, seals, and glyphs, but only three for quints.
-Each summoner is given
-You may freely choose any rune page at champion select, but you may not change their contents as you can with masteries.
When Should I Buy Runes?
That, my friend, is up to you.
However, my advice is to wait until Level 20 before unlocking any runes. Why? Because as soon as you can buy Tier 3 runes, you won't be satisfied with your old Tier 1 and 2s. While you can use the rune combiner to recycle your old ones, the fact that the result is random rarely produces something useful.
Moreover, I feel that your IP is better spent on new champions, at least in the beginning. You'll inevitably find new characters you love to play, and I encourage you to master as many different kinds as possible. Besides, since the different character archetypes call for different rune sets, you may not wish to invest hard-won IP into characters you'll end up dropping anyway.
If you've found a character right away that you really love and are antsy to buy runes to match, consider the fact that Tier 1 runes are very cost-effective. For example, look at Fortitude (health) seals. Tier 3 gives you 5.35 HP per rune and cost 820 IP each. By comparison, Tier 1s give you 2.97 HP, but cost only 60 IP. That's over half the effect for only 7% of the cost!
Which Runes Should I Buy?
Again, this is up to you. Of course:
-Buy runes to complement the characters you play. Obvious enough. Buy stats that either accentuate a champion's strengths or mitigate his weaknesses.
-We usually buy runes with the focus on the early-game. The effect of runes is generally most important at the beginning, when everyone has low stats and few items. Some rune sets retain their significance through the whole game. Consider at which game phases your character is strongest.
-Scaling runes outstrip flat runes
Where Should I Start?
Most would agree that there are several "best" choices in each category. Here's a list:
Desolation (armor penetration) - 14.94 total ArPen for 410 x 9 = 3690 IP OR Insight (magic penetration) - 8.55 total MPen for 410 x 9 = 3690 IP
By far the best-in-slot for vast majority of characters. Damage penetration is a tough stat to buy in-game, but it helps your DPS significantly. Notice that few characters have scaling Magic Resist per level, so reducing their base 30 MR to zero is relatively easy combined with masteries and early-game items. Because of the math behind it, flat damage penetration is more valuable the less defense the enemy has. Match them to your damage type.Alacrity (attack speed) - 15.3% total atkspd for 410 x 9 = 3690 IP
Some champions may prefer to have more attack speed over penetration, e.g. to proc more on-hit effects or shreds. Most junglers will be able to clear the neutral creeps much faster with these.Seals
Vitality (health per level) - total 175.0 health @ lvl 18 for 410 x 9 = 3690 IP
A decent defensive choice. Notice that HP will protect you from both physical and magic damage.Defense (Armor per level) - total 24.3 Armor @ lvl 18 for 410 x 9 = 3690 IP
A fine choice if you're not planning to build any armor items. Because of how armor stacks, you may be better off with more health.Clarity (Mana Regen per level) - total 10.5 MPR @ lvl 18 for 205 x 9 = 1845 IP
These will greatly help your MP management. For many characters, this may be all you need to avoid buying a mana regen item. And they're cheap, too!Glyphs
Force (Ability Power per level) - total 27.5 AP @ lvl 18 for 410 x 9 = 3690 IP
Ability Power adds damage to most abilities (surprise!). If your character has good ability power ratios, these are most likely your best option. A natural fit for most caster-types.Focus (Cooldown Reduction) - total 5.85% CDR for 820 x 9 = 7380 IP
Some characters greatly benefit from getting as close to the 40% CDR cap as possible. These can help you get that last little bit you need. Since the new mastery trees have more CDR available compared to Season 1, you might not find them as necessary as before. They're still a decent choice for most any character. Scaling CDR glyphs (Celerity) are better choice ONLY if you're not getting any other source of CDR (you're likely to go over the cap).Shielding (Magic Resist per level) - total 24.3 MR @ lvl 18 for 205 x 9 = 1845 IP
Extra MR means more survivability. Remember, the less you have, the more valuable it is. For comparison, this rune set offers the same amount of MR as Mercury's Treads, the first defensive item most tanks will buy. Squishy AD carries will especially appreciate these.Quintessences
Fortitude (Health) - total 78 HP for 2050 x 3 = 6150 IP
Flat HP Quints used to be the default choice for nearly every champion in the game. They've been nerfed since then, but they're still a decent choice. Every single character in the game will benefit from them.Strength (Attack Damage) - total 6.75 AD for 1025 x 3 = 3075 IP
Physical carries will love these. The extra damage has a very noticeable impact on your harassment potential, particularly if you have long range on your auto-attacks. AD Quints will also tremendously help your last-hitting.Potency (Ability Power) - total 14.8 AP for 1025 x 3 = 3075 IP
15 AP will give many mage champions a bit more oomph behind their abilities in lane, more so if you've invested in the Archmage mastery or Rabadon's Deathcap.Swiftness (Move Speed) - total 4.5% MS for 2050 x 3 = 6150 IP
If your character is focused on chasing or kiting (e.g. Udyr, Singed), these will obviously help. Pair them with the Initiator or Swiftness masteries for increased effect!Avarice (Extra Gold / 10 sec.) - total 3 Gp10 for 515 x 3 = 1545 IP
Support characters just loooooove gold-over-time effects and don't care much about the other stats quints can give. If you find yourself playing such characters a lot, they're a great investment. This set gives an extra 180 gold per 10 minutes, or 810 over a 45-minute game. That's ten extra wards!Anything Else I Should Know?
Just one thing. One-and-a-half.
If you feel cramped by having only two rune pages, I highly recommend the seven-page bundle. It costs about $20 USD, which is relatively expensive, but on the other hand you'll never be annoyed by your rune book again. It's the only "functional" thing you can buy for RP. Don't bother with buying pages one-at-a-time; it's really painful to shell out 6300 IP for an empty page.
If you do buy the bundle, think of a way to keep your runes organized. Use a system that will help you quickly remember each page's contents in case of last-second switches. I personally name my pages by each color's stats, using lower case letters for scaling runes. That way, I can see right away that my "APen/mpr/mr/AD" page has Armor Penetration marks, scaling MP Regen seals, scaling MR glyphs, and flat AD quints. Easy!
So there you go. As always, feel free to share your rune preferences in the comments. :)