Loops can be great fun and provide inspiration
There are different types of loops - the most popular formats are plain WAV files, ACIDIZED & APPLE LOOP files, REX files and MIDI files
Acidized clips are already programmed to match the tempo of your project so you should just be able to drag them in and you are done
Others may need to be dragged in and then "groove clipped" to get them following tempo of the DAW
If you change a clip (eg stretch it to fit the tempo) then the quality of the clip may sound weird and/or "lot fi". Some DAWs use a low quality "live preview" algorithm to play back the loop
To hear the best quality you need to use an "offline rendering algorithm" - the easiest way to do this is to "bounce clip"
There is also a format call "REX" which also contains some loop information but in a different way than ACID loops - rather than "stretching" the audio a REX file slices up the audio at the transients and then plays it back - so their tolerance to speeding up/slowing down is better (as there is no stretching) but on less percussive sounds it may sound a bit odd due to the lack of transients
Loops can also be told to play at different pitches by adding change markers in your DAW
I had a play with some loops and this tracks is solely loops from different places that I put together:
https://soundcloud.com/boydiemusic/loopy-groove When you get used to using the, you can really "slice & dice" them up and put them together to create unique pieces - I remember Viscount Kramer creating a fantastic sax solo in a track by cutting up loops and putting them back together again
It is definitely a tool worth having a play with