The technique of matching the tempos of two tracks for mixing them, and getting them in sync and in phrases. Done using the pitch control on the Turntable or CD deck, which may or not use a Key Correction(a.k.a. Master Tempo) function to keep the tracks in tune.
BPM[]
BPM stands for beats per minute. Each genre of music has its own BPM range for most songs within that genre. However, most songs have a fixed BPM setting, while it is rare to hear a song without a fixed BPM. To get two songs beatmatched, both of their BPMs need to be equal, with each beat from 2 songs playing "on top of each other."
Old School Beat Matching[]
Before advances in technology, DJs had to beat match what they heard. Beat matching was done on turntables. Turntables meant for DJing had pitch controls on them. The pitch control adjusted the speed (BPM) of the song, itself. The pitch control setting was usually set at +/- 8% or +/- 16%. These settings adjusted the BPM according to the song's base BPM. For example, if a song had a base BPM of 130 BPM, and the pitch control was set at +1%, BPM the song would be playing at would be 131.3 BPM. The goal of beat matching is to get the cued song and the song being played at the same BPM and in sync. "Old school" DJs use the pitch control and the beats that they listen to from the cue and master tracks in order to beat match.
Tapping[]
In between the analog and digital era of music, there was a technique known as tapping. Tapping was used to get an approximation of a song's BPM. The tapping device was typically installed within certain high end mixers, like the Pioneer DJM 400 or DJM 500. The device would automatically calculate the BPM approximation based on the rate that the DJ presses the "tap" button.
Automated Beatmatching[]
With advances in technology, it is common to see most high end mixers or CD decks with an automatic BPM counter. For example, the Pioneer CDJ-1000 has an automatic BPM counter. It is also common for mixing programs, running on computers, to have some kind of an automatic BPM feature.