I will copy nice info I found in Telarapedia about instances, maybe it will help you.
Instances are special areas in Rift: Planes of Telara where your group or raid party is able to interact with a dungeon (or other 'area') privately; that is, without interference from other parties or raids.
* Most of the game will not be instanced and can be traveled across seamlessly without seeing a load screen, with only dungeons and high-level raids being a definite possibility for instanced areas. The server technology also allows for instances to change over time depending on player actions.
* There will be both group and private instances.
* There will be instances at end-game with different sizes. 5, 10 and 20. 
* Some instances, such as Deepstrike Mines "scale" to some degree. If you come back at a later level, you will have a different experience.
o Every dungeon will have 2 versions, a normal one for its original level and an expert version that expands the story and is intended for level cap. For level 50 instances, the expert version would be what is referred to as 'heroic' in some other games.
* Raid instances will have lock out timers
* Standard difficulty dungeons do not have a lockout, and can be reset immediately, Expert difficulty will be on a daily lock out timer. 
* Rifts can appear in dungeons. 
 Known Instances
* Darkening Deeps
* Deepstrike Mines
* Iron Tomb
* Overwatch Keep
* Realm of the Fae
* Spring Terrace
I think there will be 10 and 20 man raid instances, and looks a very similar system like wow has now in Lich King.
Im looking all days for new info about pve raids but atm there aren't a lot of info about it. Edited, Nov 25th 2010 6:36am by nimthea