Other
Cal Poly Pomona: Igneous Rocks
If you need a technical summary of igneous rock classification, then this resource is worth checking out. It has several different diagrams to help you visualize the classification process.
Other
Geology.com: Rocks: Photos of Common Intrusive and Extrusive Igneous Rock Types
Explains the difference between intrusive and extrusive igneous rocks and provides pictures and descriptions of common igneous rock types in each category.
CK-12 Foundation
Ck 12: Earth Science: Intrusive and Extrusive Igneous Rocks
[Free Registration/Login may be required to access all resource tools.] Describes the features of intrusive and extrusive igneous rocks that cool slowly and rapidly, respectively.
CK-12 Foundation
Ck 12: Earth Science: Intrusive and Extrusive Igneous Rocks Study Guide
[Free Registration/Login may be required to access all resource tools.] Summarizes the key points about how intrusive and extrusive igneous rocks form. Includes a few questions to check for understanding.