Official rules for the tournament IPL

The official rules for the tournament are here.

There are five ways that a franchise can acquire a player. In the annual auction, buying domestic players, signing uncapped players through trading and buying replacements.[18][19] In the trading window the player can only be traded with his consent. The franchise will have to pay the difference between the old contract price and the new contract price. If the new contract is worth more than the older one then the difference will be shared between the player and the franchise selling the player.[20]

Some of the Team composition rules are:

Minimum squad strength of 16 players plus one physio and a coach.
No more than 8 foreign players in the squad and at most 4 in the playing XI.For the 2009 edition franchises are allowed 10 foreign players in the squad. The number allowed in the playing XI remains unchanged at 4.
A minimum of 8 local players must be included in each team.
A minimum of 2 players from the BCCI under-22 pool in each team.
The players accorded "icon" status are: Sachin Tendulkar, Rahul Dravid, Sourav Ganguly, Yuvraj Singh and Virender Sehwag . The total spending cap for a franchisee in the first player auction was US $5m. Under-22 players are to be remunerated with a minimum annual salary of US $20,000 while for others it is US $50,000. Icon players are to be paid 15% more than the highest paid player in their respective teams.