The 260 balloon (pencil) is the main balloon used by twisters. Some balloon artists use only the 260. You can make pretty much everything out of this balloon. I personally like to use lots of the balloons available but it does tend to mean I carry too many.
In addition to the 260's there are also 160's,350's and 646's. These are all the same shape as the 260 but different widths and lengths.
Balloon twisters can also use the wide variety of other shapes available these include:
Rounds, Geo-blossoms, hearts, bee-bodies (321q) and slim-jims.

Balloons pictured:

Lilac Geo Blossom, 6 inch pink heart, 6 inch printed round (pumpkin), White bee body, solid yellow 160, lime green 260, silver 350.

The balloons pictured are all made by Qualatex which are a high quality manufacturer of twisting balloons. Another company worth looking into are Bellatex (named Sempertex in UK and distributed by Graham Lee).