Sentence fragments are strings of words that have been punctuated as a sentence but are not valid sentences; that is, they don't contain all the elements necessary to create a sentence. Sentence fragments are usually incomplete ideas; for example,

Although the composition of the student body has changed dramatically.

This sentence does not convey a complete idea. If these words were spoken to you, you would wait for the person to tell you the other half of the idea:

Although the composition of the student body has changed dramatically .... what?

Compare this to the following sentence:

Although the composition of the student body has changed dramatically, little variation in instructional techniqueshas been apparent in Australian universities until recently.

Notice how this revised sentence presents a whole idea, made up of two halves. These example sentences below also consist of two halves put together to make a whole:

Because the young male population was engaged in the war, women were recruited into the domestic labour force.

Solids turn into liquids when they are heated.

Notice that one of the halves or clauses of the sentences above begin with words like 'although', 'because', or 'when'. This half is putting a condition on, or adding information to, the other half. It is a dependent clause: it can't stand on its own, but depends on another half to complete its meaning. Dependent clauses by themselves are sentence fragments; therefore, you must make sure that you join dependent clauses to independent clauses.