Hi,
If you are writing a post on someone else's blog, then you are a guest blogger. A blog owner may allow guest bloggers to post on his website for a few reasons. One may be that he is too busy to post regularly himself. Or he invites guest bloggers to add variety to his blog. Sometimes he may be able to get an established blogger to be a guest blogger to get more traffic.
If you are writing a post on someone else's blog, then you are a guest blogger. A blog owner may allow guest bloggers to post on his website for a few reasons. One may be that he is too busy to post regularly himself. Or he invites guest bloggers to add variety to his blog. Sometimes he may be able to get an established blogger to be a guest blogger to get more traffic.