In music, "beat" refers to the pulse and tempo of a particular passage. The time signature indicates the number of beats per measure and the length of the note that gets each beat.
In 4/4 time, there are four beats per measure, and a quarter note gets one beat.
In 3/4 time (like a waltz), each measure has one strong beat followed by two weak beats.
Note that in "cut time", written for convenience the same as 4/4, there are really only 2 beats per measure.
Rock and roll typically has a "back beat": a strong pulse on the 2nd and 4th beats of each measure, providing some syncopation.
- "It's got a back beat, you can't lose it, any old way you choose it." - Chuck Berry, Rock'n'Roll Music
The "blues beat" is typically signified as 6/8 - although one might argue it is indistinguishable from 3/4 time, the placement of the beats make it very different.