Gluten is a protein found as a major component of wheat, rye and barley. Gluten can be separated from the starch and is useful to make dough more elastic and bread made with additional gluten is more firm, remains fresh and in its form.
Flour is graded by its gluten content, with bread flour having the most, and pastry flour, the least. The more widely-available all-purpose flour is a compromise between the two, having an intermediate amount of gluten.
Gluten develops its firmness as dough or batter is "worked". In a relatively dry dough, as is used in most breads, this development of structure causes the dough to be noticeably firmer as kneading progresses. In looser doughs, such as those used for focaccia or certain thin-crust, high-temperature pizza doughs, working of the dough is necessary but its effects aren't apparent until the dough is baked.
For most pastries, the development of gluten is undesirable. Pie crust and sugar cookies are common examples. Recipes of this type often warn against the toughness that results from over-working the dough.
Gluten is the main component of Seitan, a versatile meat substitute.