What are passing tones?

Can someone tell me what passing tones are and how to apply them?

Thanks people, you've all been a big help.

Passing tones are notes that are out of key (accidentals) that lead to another note.

So let's say for example, you play the note "C" (just the note, not the chord). Following this C, you would play a "D". A passing tone would be a note played inbetween these two notes that leads to the 2nd.

So a C# played inbetween them would be a passing tone.

When you apply this with chods, you get the same idea except on some note in the chord. The most common is the bass.