TPB : Ticks Per Beat. The number of rows the patterneditor will move down during 1 beat. Higher TPB will give you more control over timing at the expense of screen-estate.
The word Tick is used to indicate the overall position in your song. As stated, a Tick is represented by a single row in a pattern. Simple as that. Every row within your pattern editor is equal to one Tick, and in sequence editor default one bar is equal to 16 Pattern rows, so to 16 Ticks. If you want to change this, read Understanding Bars in the Sequence Editor. The best way to explain the concept 'Ticks per Beat' is the probably the following example:
Let's say you compose a trance song and assign 1 bassdrum hit every 8 rows in the pattern editor of your bassdrum generator. You set the BPM value between 120 and 150 and the TPB value to 8. Your song is now oriented around an 8 'ticks per beat' grid (the pattern editor resolution). The BPM multiplied by 8 will then give you the number of rows for one minute of music.