BPM
Beats Per Minute
A number showing how many beats there are per minute, indicating the speed of a track. At 120 BPM, that is 120 beats per minute, or 2 beats per second. The larger the number, the faster the track. Whereas tempo is "the speed itself," BPM is the value that measures it. When connecting two tracks, you first match this number.
How to count BPM
BPM is a count of how many beats fit into one minute. To check it yourself, clap along to the track, count how many claps you make in 15 seconds, and multiply by 4. If you clap 30 times in 15 seconds, that is 30 × 4 = 120 BPM.
DJ software and gear analyze the track and show its BPM for you. But for tracks whose speed changes partway through, or tracks whose beat is hard to pick out, the display can show half or double the real value. Check the displayed number by ear before you rely on it.
Matching the BPM of two tracks
When mixing two tracks, you match the BPM of the next track to the one that is playing. You use the tempo slider: the track speeds up or slows down as far as you move it. Once the BPMs match, the beats of both tracks advance at the same interval, so they stay together when overlapped.
Matching the BPM does not line up the beat positions, though. You close that gap with NUDGE or the jog wheel. Doing both steps by hand is a beatmatch; leaving them to the machine is SYNC.
What happens when you change BPM a lot
When you raise or lower the tempo, the pitch normally moves with it, just as a voice gets higher when a record spins faster. A small change goes unnoticed, but a big one changes the feel of the track.
MASTER TEMPO avoids this by changing only the speed while keeping the pitch. Even so, the further you move from the original speed, the less natural it sounds, so the basic approach is to mix tracks whose BPMs are close to each other.