We can use different stems in order to write more than one voice in a single staff. The next example may help:
In this example we are able to write 3 voices with different rhythms in a single staff by using different stems.
In these 2 chords:
Some notes are moved to the right to avoid collision with other stems but they should be played at the same time.
Going back to the previous example, note that the E has been moved to avoid collision with the A (circled notes). Both notes are to be played at the same time.