You use :System.cursor_item or :System.cursor_Block to determine where the cursor focus is.
You use trigger_(item or block) in triggers where the cursor may not be in some other location. For example, in a post-query trigger, you must use trigger_record to determine the row on which the trigger is running. Cursor_record is usually the first row in the block during query processing.
In validation triggers, you need to use trigger_item rather than cursor_item, since validation triggers can run when the cursor is not necessarily in the particular field where the cursor is located.