Vertical black bars called bar lines divide the staff into measures. (The UK prefers simply 'bars')
This staff has been split into two measures.
Time signatures define the amount and type of notes that each measure contains.
The first measure is in 4/4 time and the second measure is in 3/4 time.
The first measure (4/4) contains four quarter notes (called 'crotchets' in the UK).
The second measure (3/4) contains three quarter notes.
You can see some examples of pieces written in these time signatures on 8notes.com -
Sheet music in 4/4 time,
Sheet music in 3/4 time.
Next, we will discuss non-quarter-note time signatures.
6/8 time contains six eighth notes (called 'quavers' in the UK).
3/2 time contains three half notes (called 'minims' in the UK).
You can see some examples of pieces written in these time signatures on 8notes.com -
Sheet music in 6/8 time,
Sheet music in 3/2 time.
This chart displays all of the time signatures that we discussed.
Sheet music in 4/4 time,
Sheet music in 3/4 time,
Sheet music in 6/8 time, and
Sheet music in 3/2 time.