The useBean tag will check the specified scope to see if the bean already exists, if not, it will instanciate it, create an object variable that points to it, and then bind it to the specified scope for you. All with one line of code (tag).
[appended] By the way, if you want to know exactly what the difference is, create a JSP page that instanciates two beans -- one by hand, the other with useBean. Run it. Then go look at the generated source and compare the code for each. [ July 14, 2005: Message edited by: Ben Souther ]