To try to clarify a bit more:
A preamp will take a mic level signal, which normally will be too low to be used by most audio equipment to a higher level signal, it amplifies it. They usually will also supply phantom power for condenser mics.
An AD/DA converter takes an audio signal and converts it to digital.
A sound card (by itself) is a device that helps process digital audio signals inside your computer. Most soundcards have some degree of AD/DA built in as well.
AN audio interface is generally a device that combines two or all three of th above. Almost certiainly you will get AD/DA converters and the soundcard funtionality that lets the computer manipulate the audio in digital format. Often you will get at least a couple preamps built in as well.
Last, quality of the various items above tend to be proportional to their "bundled" status. Entry level home recordists will usually want a all in one packatge with preamps, converters, etc all in one package. But stand alone preamps, converters, etc tend to be, item-by-item, more expensive and higher quality.
Daav.