Rhythm in music is indicated in several different ways. Firstly, time signatures and bar lines are used to measure out regular units of time and describe how those units of time can be divided up. Secondly, different shaped notes are used to indicate how much of one measure of time the note is to take up. Third, rests are used to indicate when no notes are to be play, and for how long. The combination of these things (and others) results in almost any rhythm imaginable being able to be accurately notated.
The diagram below shows the most common note shapes and rest with their relative durations. Reading rhythm is of course much more involved than learning the different note and rest values however this makes a good introduction;
To learn more about reading rhythm in music notation follow the links below.