The first screen of the game lets you choose between 2 modes :
Easiest mode recommended for beginners and get familiar with the game. There are a lot of Extends, no time limit and you can continue endlessly from where you died. Furthermore, on Dreamcast and AES versions you can get an extra Continue by collecting all the Big Diamonds in a stage (it balances the fact that you have only 4 credits on those versions).
Harder Mode for experimented players. The intro cutscene is removed and there is a timer that can be extended by collecting Diamonds and Time Extend items. There is no One Up items anymore, they are replaced by Time Extend items. The only way to get a life is to collect all the Big Diamonds in a stage. At last, if you lose all your lifes and use a Continue, you will respawn at the beginning of the level.
The third line is to change the buttons mapping (joystick or pad for MVS and AES versions, on Dreamcast it is a real option menu to map all the buttons as you like).