Pre-season will happen in between S3-S4 to test the major changes that happening between seasons. With S3 ending 11/11/13 we should see pre-S4 happen very shortly after.
What will be changed
General New gold balancing, turrets will give local (near it when destroyed) gold, Baron buff will be getting re-worked eventually, dragon will scale to late game
Jungle New mob, new monster scaling, new items, new effects on old items, new gold income possibilities
Vision Ward limits per person (currently 3 green, 1 pink), pinks can take 5 hits but are always visible (their price is reduced).
Champions general balance changes, utility spells will get coeffecients to the effects themselves. i.e. Janna shield's AD bonus has a +0.2AP bonus, Lulu's passive will have a +0.3AP bonus. (numbers are random not confirmed). New champs of course, champs will NEVER BE REMOVED from the game ever. at all. Will probably see a lot more VU's as well, Nasus has one coming!
Map Bushes are being worked with, with the intention of being able to get vision in each bush with a single ward
Items A lot of items will be getting new effects, mainly gold income items. New support and jungle items confirmed. and TRINKETS, a new item type with its own slot. These will be free and available from the start. 3 confirmed so far, all having to do with vision, 1 drops a ward, 1 is a CV clone, and another is a stealth detection and ward 'jammer'.
Summoner Spells CV and Revive will be removed, revive may come back later if a rework is found.
Runes new rune inc, possible examples include hybrid defense runes and rebalanced quints.
meta jungle will be hugely changed, new champs will be able to jungle. With the new rune discussion lvl 1 armor across the board has been called into question. It has been speculated that carry jungles will be viable. New support items also synergize amazingly with kill/damage lanes, so new viable supports will be coming (ashe anyone?). As a note as well, global passives have been almost completely killed off due to Janna's becoming short range and we will more than likely never see a champ with a global passive again.
That's about it, for more information: http://www.surrenderat20.net/ or
http://na.leagueoflegends.com/en/news/game-updates/features/preseason-draws-closer