More pixels means more detail and clarity, and this means a better quality picture. The simplest have around 350,000 pixels, the highest resolution over two million. This allows you to frame scenes so that you see exactly what you'll get when you take the picture. It'll also allow you to check a picture as soon as you've taken it, so you can check what you've taken, and delete it straight away if it hasn't turned out the way you wanted it to. The more memory you have, the more pictures you can store on your camera before you need to transfer them to a computer.
Most cameras use memory cards .
As for which type to choose, it's more a matter of what comes with the camera you want. The problem with that is that storing high quality pictures needs more storage than you get with a floppy disk, and so there will be necessary compromises in terms of image quality. Expect to see cameras with a built in Iomega Clik making increasing appearances. The Clik is a disk drive with a small removable disk that stores 40Mb of information. Clik disks have the great advantage of being relatively cheap.