A twip is equal 1/1440 of an inch. As stated, a twip is also 1/20th of a point, with a point being about 1/72 of an inch. For those of you using the metric system, a twip is 1/567 of a centimeter. Note that all measurements are approximate, as the size of a twip can change due to screen resolution.
Twips are indeed used to denote positioning of forms in Visual Basic. Twips are, in fact, the default measurement unit, and are used in everything from size of an item, to coordinates of a shape.
Twips, like pixels change size according to screen resolution. However, unlike pixels, twips express an absolute size when printed.