What is a loop?

a loop is a groove in a phonograph record that has been cut back onto itself after only one rotation
that is, it goes nowhere and it will play forever if you let it

What is a "locked groove"?

almost every record has a locked groove
a typical record consists of 1 groove that plays from the outside toward the inside of the record
unless the groove is cut back onto itself at the very end, the needle would be allowed to go onto the label
(this may happen when the locked part gets worn out)
almost every record consists of simply two locked grooves
one for each side

How are loops useful?

a loop will repeat almost perfectly if the audio is some rythmic approximation of 133.33 bpm in 4/4 time
that is, faster if the meter uses a larger denominator; slower if a lower one
in the time it takes for the record to make one rotation, exactly 1 bar of music/sound can be played
loops can therefore be made to have 1 bar of beats, music or sounds that a dj can repeat while mixing with
other records or loops

Why do loops suck sometimes?

because either:
a. the producer was a bonehead and used some rather imaginative formula that does not resemble the one above; or
b. the producer was a bonehead and recorded something that sucks; or
c. the producer was a bonehead and recorded the loop on some medium where the
....speed of the recording process is unrelaible (i.e. cassette) ;or
d. (and most likely) the mastering engineer is not a robot and is physically and neurologically incapable of cutting a groove back ....onto itself precisely after one rotation from when the groove was started
that's right folks, there's luck involved

What are those damn pops and screaches?

that is the sound the stylus makes when it cuts back into the groove too early or too late
if the mastering engineer is really lucky, he or she will "sneak" the stylus back into the groove during a relatively quiet millisecond. a moment in the music/beats that contains little bass and overall not much sound
then the cutback will be almost undetectable by ears
but when the cutback lands ontop of a moment of pre-existing audio splendor, then the result is some distortion as
the stylus mangles the laquer in ways it was never meant to be cut

When are you guys gonna stop making records with loops in them?

shortly after the last dj uses one for the last time