When you create a new post, Blogger will start automatically saving it for you once a minute, without interrupting your typing. When it does this, you'll see a little note next to the "Save" button, like this:
If you want to make sure the saved copy is completely up to date, you can click the "Save" button yourself. You'll remain on the posting form and can continue typing, since the saving happens in the background. If you're done working on the post for now, you can either click "Publish Post" to convert the draft into a post, or click "Return to list of posts" if you want to leave it as a draft and go back to the Edit Posts page.
This autosave feature means that you have a safety net for the cases when you're working on a new post and the power goes out, or your browser crashes, or some other calamity occurs. All you have to do is log back in to Blogger and find your saved draft on the Edit Posts page. You can even do that from a different computer.
Note: Autosave does not occur when you are editing posts that have previously been published to your blog. This keeps your readers from seeing your edits until you're done with them. When editing a published post, you can click "Publish Post" to republish the changes to your blog, or click "Save as Draft" to unpublish the post and turn it into a draft.