Literary Agent
A literary agent works with you to develop your manuscript so it's ready to submit to acquiring editors. For nonfiction authors, for example, this will include helping you construct a book proposal, which is essentially a marketing/sales pitch. For fiction authors, this will include helping you perfect your story. Once an acquiring editor offers to publish your book, your agent will negotiate your book contract. The agent will likely be in charge of disseminating your book advance and royalties to you and will generally look to see how you can grow your career.
Literary Lawyer
Although a lawyer can represent you in your book deal, a lawyer's role is a bit different. A publishing attorney likely won't help you develop your book proposal or manuscript. Rather, they come into the fold to negotiate your contract when you have a publication offer. Attorneys can also help you review your manuscript for any legal issues, such as copyright or trademark infringement, libel, or right of privacy violations, among other things. They can also help draft permissions licenses, so you can legally use copyrighted material.