How is Git hash constructed?

Question

What kind of properties make up the hashes in Git? I realize that it may differ depending on the object type (commit, tree, etc.).

For example, how is the commit hash made? Does it involve the commit message perhaps and the changes?

Answer 1

From Pro Git:

The output from the command is a 40-character checksum hash. This is the SHA-1 hash — a checksum of the content you’re storing plus a header

There's more detail in that chapter.