There are three completion types:

Logical name

Shortcut

Basic

Ctrl+Space

Smart (SmartType)

Ctrl+Shift+Space

Class Name

Ctrl+Alt+Space

Pressing the same shortcut twice results in 'second' completion, which may give more variants.

You may choose item in completion list not only with Enter, but also with variety of other keys: space, dot, colon, semicolon, '(' etc. Basically this will result in inserting that character after the completed identifier, but sometimes more wil be done. For example, finishing Java method call with '(' will, of course, insert also ')'.

Choosing item with Tab has special meaning. In this case, if you are inside identifier, it will be overwritten completely instead of just inserting the item string into caret position.

Java

Basic

Smart

Basic & Smart

Class Name