Riot has released a website along with a short video previewing some of the major changes coming into the preaseason.
On top of extensive summoner and mastery changes, Riot has also made large changes to the jungle, support role, warding, and dragon. Dragon will now scale based on game time to allow it to be an important objective all game long. Wards are now limited and varied to allow for more in-depth strategy and play. Supports and junglers now earn more gold to sustain relevance lategame similarly to their farming counterparts.
The site features an interactive map replica of Summoner's rift with clickable icons for more details. Here, you can see the more in-depth changes such as the new creeps being added to the jungle to help increase the gold and experience supply.
Source - Riot Games