A word of caution
Please don't go editing object in your editor. You can corrupt your Git repository if you aren't careful. It's worth the time to learn to use git grep. It's really not all that different than grep, and it's much faster. To search through commit messages, take a look at git log --grep.
Underneath the hood, git has a concept of objects. Objects are generally made up of a header plus some data. File content gets stored as a blob object. Tree objects contain filenames and point to blob objects that represent the files, and tree objects that represent other directories. Then there are commit objects which record the log message and and point to the tree object that represents the appropriate tree state. There are also annotated tag objects as well, which generally point to a tag placed on a commit.
The online book has some information about the different kinds of objects and how to view them and also includes some details on the format used to store the objects.
Keep in mind, what you are looking at are loose objects. There are also object contained in pack files, which can have a different format.
The Git User's Manual also has good information about the object database.