Castlevania: Portrait of Ruin is a bit tricky when it comes to endings. If you don't know exactly what you're doing, you may get the 'bad ending' without knowing what you did wrong. Anyways, for the sake of posterity, here is how to get the various endings.
First off is the bad ending. The bad ending is where instead of a credits screen, you get instead 'Game Over'. You get this by killing Stella and Loretta, the evil Vampire twin sisters. When you get this ending, it means that you cannot access Richiter, Sisters and possibly the Old Axe Armour Modes.
This is where, instead of executing the twins, you get Charlotte Aulin to purify them with her special spell 'Sanctuary'. This is a lot more difficult than it sounds, as for some reason, 'Sanctuary' takes precious minutes to fully power up and cure the girls of their vampirism, and during that time the girls have the posterity to kill you amid your attempts to help them. You can find the spell once you have gained Charlottes 'Toad Morph' move in the area of Sandy Grave - it's in the room with the rolling boulder.
The Good Ending allows you to play in 'Sisters', 'Richiter' and 'New Game +' Modes once you have defeated True Dracula. It also allows you to play the game on any mode (including the main mode ('Jonathan' mode)) on 'Hard'. The difficulty ratings are level caps, and the modes span between 'Moderately difficult' (level cap of 50), 'Difficult' (level cap of 25), and 'Insanely Hard' (with a level cap of 1). A piece of advice, however, is only to try the Hard mode with a level cap of 1 if you have strong mental fibre and can withstand the dying in one shot, which one would assume would happen a lot in a level 1 capped mode. Otherwise one may end up throwing their Nintendo DS out the window - and we wouldn't want that now, would we?
The Vincent Endings
Halfway through the game, our merchant friend Vincent gets bitten by a Vampire. That ought to show him for overcharging us, eh? Either way, you can gain access to a purification spell (the move 'Sanctuary', one of Charlotte's spells), but it's entirely up to you if you want to cure him or not. Some people just leave him as payback for charging so much, others get sick of his moaning and wallowing in self-pity about his imminent change to being of the blood-supping persuasion, while some strange people feel genuinely sorry for him. No idea why.
Whether you choose to cure him or not, this has a slight effect upon the ending, and these endings occur in both the bad ending and the good ending. The funnier of the two is, of course, leaving him to become a vampire.