Certain notes can be written in different ways, although they sound the same when played. Such notes are referred to as enharmonic notes. E.g. the note G# sounds the same as the note Ab , so G# and Ab are said to be enharmonic notes. There are several pairs of enharmonic notes that you should become familiar with.
Enharmonic notes are frequently written in chromatic passages, sequences of notes that move up and down by semitones (or half-steps).