In many ways sim turns is similiar to normal... the main difference is in how you issue orders to your troops and how they move during the turn. You tell them to do something. But they don't do it right then. Instead you hit end turn every one places their orders and then they are executed in "days" each game turn or month being broken up into 30 days. Thus the ships move 1 space or execute 1 command every day.
This is most imporant when trying to intercept enemy fleets or timing things like blowing up a star to take out enemy fleets. Or for that race to see who makes it to the warp point force.
Also in some mods where you can get more than 30 move points on a single ship the ships in sim moves will only move upto 30 spaces and the rest of the movement will be lost. Result of 1 move 1 day rule. 30 days = max 30 moves.
There are no doubt some difference I missed but those are the ones that come to mind right off the bat.
Can always check out the newbie faq below as well.

Darn those faster posters!
[ April 09, 2003, 02:20: Message edited by: Cyrien ]