Guest blogging involves writing blog posts that are published on other bloggers' sites. It benefits all parties - guest bloggers gain exposure to new audiences and links back to their site, host blogs get fresh content, and readers see a variety of perspectives. To start guest blogging, bloggers should read potential host sites, provide credentials, offer quality content on topics relevant to the host's readers, and start by pitching to smaller blogs to build a portfolio. The goal is to provide value to readers rather than self-promotion.