This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
Network calls are expensive especially if you have a lot of them. For example, suppose I need to get 20 items a table in the database. And assume my database and application server are on different machines. I could send 20 SQL queries by primary key to get the data. This is 20 round trips. Or I could send one SQL query to get all 20 things. This is one roundtrip.