Watch the Measures online guitar lesson by Rich Maloof from Acoustic Guitar For Beginners

Like the majority of rhythmic feels in popular music, the rhythms in this course break down neatly into blocks of four beats each. You could even count along instead of strumming or tapping: 1 2 3 4, 1 2 3 4, 1 2 3 4. Each four-beat block is referred to as a "measure" or "bar" of music.
When four evenly spaced beats fill up a measure, each beat is a quarter note. So if you strum down on each number ( 1 2 3 4 ) you're strumming quarter notes. If your arm is swinging steadily and you catch all the strums on the way back up as well (1 and 2 and 3 and 4 and), each beat is an 8th note.