Loops are usually created in a sampler. An audio file is dropped into the sampler device and "Start" and "Stop" points are set using a pair of dedicated controls. To create a traditional loop, you would place the start point on the "one" count of a bar and the stop point at the end. Moving these points off of the bar starts creates an unpredictable, uneven sound, not unlike the sound of an old vinyl record skipping due to a scratch.
Regular loops need to be locked to a song's tempo to make sure they stay in time as the song progresses, but there's no reason you can't use multiples of the tempo to create peculiar backings or atmospheric effects. Halving the tempo can create slow, undulating sounds that retain the original feel of the song, but splitting it into quarters or eighths can create long, atmospheric tones.
Multitap delay effects encapsulate a number of delay or echo units, each of which can be set to loop at a different rate. By feeding a music loop into one of these units, you can create overlapping textures and syncopated rhythms. The feedback controls also allow you to control the number of repetitions, so you can create long, evolving tails, or short, staccato bursts, much like the skipping record effect.
A popular motif in loop-based music is using a shortening phrase to build tension before a chorus or new section. Create a four-bar loop and insert four instances of it, then trim the loop length to two bars and insert another four instances. Continue the process until the sample is only a fraction of a second long. This effect is similar to a skipping CD, only you can control the rhythm.