Let me start by clarifying something for you, loops and plug-ins are entirely different. A audio loop (there are also MIDI loops available) is a recording of a short performance, usually 1-4 measures, of drums, bass, guitar, or any instrument (vocal loops are also available). Loops are created in several standard formats. WAV, Rex2, and Apple Loops are three of the most popular, and most DAWs can handle a variety of formats, and time-stretch the loops to fit the tempo of your song (check your DAW's documentation to see which formats it's compatible with) . So, for example, a good drum loop collection — usually drum loop sets include variations and fills all based on the same groove — makes it possible to put together a full-length drum track, by editing the loops together. And, yes, loops can be used in more than one DAW, as long as they're compatible format wise. In your case, GarageBand's Apple Loops are supported by Digital Performer, so you should be able to easily use them in both programs.
A plug-in is a software program, usually either a signal processor or a software instrument, which you can open inside a DAW like GarageBand, Digital Performer, Sonar, Studio One, etc. An important distinction is that there are two types of plug-ins, those that are built into the DAW, and those that you buy separately from independent plug-in manufacturers. Only the latter, the third-party plug-ins, can be opened in different DAWs (assuming the format is compatible, of course). GarageBand and Digital Performer both support Audio Units on the Mac (I'm assuming you're talking about DP on the Mac, not Windows) so third party plug-ins in that format could be used in either program. Hope that helps.
