I've heard of people claiming a lot of dependents on their w2. Besides claiming, spouses, children and relatives, what else can I claim as a dependent?

[AnnetteB]

You should only “claim” the number of exemptions for your W-2 withholdings that you expect to claim on your tax return.  This means claiming yourself, your spouse, and any dependents. 

Claiming exemptions for your W-2 withholdings by filling out Form W-4 changes the amount of Federal income tax that is withheld from your paychecks.  When people artificially inflate that number of exemptions, they may not have enough taxes taken out of their check and will owe money (and possibly underpayment penalties) to the IRS when they file their tax return. 

There is no immediate penalty for claiming more exemptions for withholdings than you will claim on your tax return, but it is not something you should do unless you prefer to pay taxes out-of-pocket with penalties when you file your return.