When we strike one string by itself we call it picking. When we play two strings at once we can call it a double stop or a partial strum. When we strike more than two strings with a single motion of the picking hand, it is called a strum. You can strum slowly or quickly. A slow strum will allow you to hear each note individually. A quick strum will sound all of the notes at the same time. The quick strum has a rhythmic impact much like a drum and allows us to use the guitar with great rhythmic effect.
There are really only two directions you can go when strumming, down or up. With the down strum you begin by playing the bass strings and move toward the treble strings. With an upward strum you begin with the treble strings and move toward the bass strings.
You can create rhythm by combining the downward and upward strums by strumming down quickly and immediately strumming up again. You can repeat this movement indefinitely because in strumming upward you again return to the starting position and are ready to strum down again.
Using tablature we can indicate strumming in a couple of different ways. One way displays all of the notes:
The other way shows the chord name above the tab and uses a special symbol to indicate the strum. We actually have two different symbols to indicate strumming, one symbol indicates a downward strum and the other indicates an upward strum. This is my preferred method for indicating strums using tab.