Cladistics is classification based on common ancestry, and uses a cladogram to place species in order that they descended from the common ancestor.
A clade is a group of species that shares a common ancestor. Each species in a clade shares some traits with the ancestor, and each species in a clade has traits that have changed.
Go to the How to Build a Cladogram page at Northwestern University. Watch this video on how to make a cladogram. You can also watch the Bozeman Science video, Cladograms.