The geometric isomers belong to the family of the stereoisomers, more precisely, the configurational isomers. So, as the name indicates, they differ in the configuration of one or more carbons. The particularity of this kind of isomerism is the fact that it involves non-tetraedric carbons, that means, carbons that do not establish 4 single bonds. Putting it more simply, it involves carbons that establish double bounds.
The classic example of this kind of isomerism is the cis and trans isomers, where the configuration of one of the carbons involved in the double bound changes. It is important to note that these isomers are not conformational ones, because the double bonds cannot rotate.
Finally, it is noteworthy that one should be careful when comparing two molecules that differ in a region that contains double bonds. In this case, the molecules are geometric isomers only if the constitutional isomerism is not applicable to the molecules. For example, in the following image the first two molecules are geometric isomers, while the molecules 1 and 3, or 2 and 3 are constitutional isomers.