This is the commonest form of graffiiti - someone writing their name or nickname on a wall in lettering one plain boring colour with little attempt of stylizing.
Throw ups take a little longer to do and consist of lettering of two colours, perhaps in rounded bubble writing that identify the graffiti artist by their street name or nickname
Stencils can be made out of cardboard, lino or other stiff material and used in conjunction with a spray can to produce repeated images. Little skill required here. A 4 yera old can use a pair of scissors and a spray can. The initial drawing may have required some talent although could have been copied.
A piece is a shortened form of 'masterpiece' and involves the use of several colours, some design, cartoom style images and skill. They also take much longer to produce.