Isomerism in Organic Chemistry is broadly classified into two types - Structural Isomerism and Stereoisomerism.

Structural isomerism is focused on the connections between atoms. In different compounds with the same molecular formula, if the atoms are connected in different ways, we have the structural isomerism.

Stereo-isomerism is more about the arrangement. The connections may be same, but how are the atoms arranged in space?
Learn more about these two kinds of isomerism in this video.