The drums set up and keep the basic feel of a song. They also drive the rhythm forward and provide endless rhythmic ideas for the other instruments through the use of accents, fills, and rhythmic patterns.
The great thing about listening to the drummer is that you have three or four different parts that you can play around or lock in with. Many musicians (especially guitarists) rely on the drummer to keep time for them, but in a good band everybody has a strong sense of time and no-one relies on anyone else. If you are not confident keeping time for yourself, work with a metronome or drum machine every day until you are confident. The photo below shows all the basic parts of a drumkit.
This kit contains three tom toms, but many drummers use only two, as these are probably the least necessary part of the kit. In fact, most drum parts can be played using only the snare drum, the bass drum and the hi hat cymbals. The bass drum is played with the right foot and produces what is often called the "bottom end" sound of the drums. The snare drum is usually played with the left hand, but the right hand is also used for certain beats as well as playing fills. The hi-hat and ride cymbals are generally played with the right hand, but once again the left hand may be used in certain situations. The crash cymbal is played with either hand, depending on which one is most practical for each musical situation. The tom toms (toms for short) can also be played by either hand. The hi-hat cymbals can also be played by the left foot and the most common sounds using the open hi hats are achieved by using a combination of the right hand and the left foot.