There are a few types of bonuses -
1) Incentive bonus - linked to performance of an employee (this is to encourage him/her to perform still better)
2) Annual/statutory bonus - this is in terms of the Payment of Bonus Act which is a minimum of 8.33% of the employees' wages as defined in the Bonus act. This is paid in respect of the previous year irrespective whether the firm made profit & allocable surplus or not, subject to the ceiling provided in the act, not for every one.
This bonus determined after working out the 'Allocable surplus' calculated in the manner enumerated in the Payment of Bonus Act is paid, normally not more than 20%, some times more also. This type of bonus is paid by large, unionised industrial houses/factories after negotiations with the unions.
3) Exgratia in lieu of bonus - certain provisions of the Payment of Bonus Act is not applicable to certain categories of firm and employees. In such of the circumstances this payment as Ex-gratia (in lieu of bonus) is paid which is not mandatory but to encourage the employees and to keep up the tradition/precedents this is paid.
4) there are also other payments which are not 'bonus' as such but generally known as - attendance bonus (to encourage full attendance, house keeping bonus (normally shared by a group collectively maintaining their houses in order, neat and tidy. Festival bonus or Inam to mark or celebrate certain occasions like Founders' day, Silver Jubilee, Golden Jubilee and so on.
You should also read many of the posts appeared in this forum by using search option and browsing links appearing on the right of your screens. Also read the bare act of the Bonus Act to know fully well.