A literary agent represents a writer and their books to publishing houses. Having a literary agent is how authors get traditionally published. They deal in the business affairs of an author's career. Authors do NOT pay for agents, the agent will take a percentage of the commissions, from the authors sellings. 

 A query letter is a pitch, summary, and selling points of your book. These letters are sent to the agents you wish to represent you as an author. 

When an author does traditional publishing it means they were offered a contract from a publishing house. Your book will be sold through booksellers and retailers. The publishing house holds the rights to your book, and in return you receive royalties.  

A royalty is the percentage of money an author receives each time their book is sold in stores or online. This is how authors will receive passive income. Royalties are found in traditional publishing and not in self publishing. 

When a manuscript is reviewed for all grammar and punctuation errors, before print. 

Comparable book titles, or competitive ones which are similar to your story. These are often used in the query letters, as a selling point for how the book could be marketed. 

The summary of a book which is found on the dustjacket. The selling pitch for readers when they pick up your book.