Nomenclature

Technical Terms:- Threads Per Inch, Pitch, Diameter, Form

Threads Per Inch (TPI), is the number of threads (Peaks) per inch length of thread. If you place a rule on a bolt and count the number of peaks (think of a thread as a mountain top or the top of a triangle) and you have the Threads per inch. The pitch is the distance between two of these peaks.

Coarse and Fine. Basically people invented threads to suit their manufacture needs. Bicycles and watch makers needed fine threads. Pipe fitters and Bridge makers needed coarse threads. You can therefore have a 1" diameter (25mm) bolt with a fine thread with 12  (TPI) or a coarse thread with 8 TPI. The coarse thread allows quicker assembly and will carry more pressure than the fine thread.

The other variable is the angle of the thread (Form).

There are numerous thread forms, named after by their inventors. BA (British Association), BSF (British Standard Fine), BSC (British Standard Cycle Thread), CEI (Cycle Engineers Institute), ISO (International Standards Organisation) Metric Coarse, Fine, and Special Pitch, UNF (Unified Fine Thread), UNC (Unified Coarse Thread), and Whitworth Threads.

You do not need to worry about it too much, unless you are a designer who has to determine the best thread to suit his need, or you are measuring a thread to find out what it is.

Another excellent site:-  Brian's Thread Page