To categorise your clips, switch to the folder view (the two buttons to switch between flat and folder views are at the bottom-right of the sidebar). Create folders with the [+Folder] button and name them. You can drag and drop your clips to move them to another folder.
From version 3.3, you can add tags, comments, search terms or misspellings to each clip. To show the tags field, use the little disclosure button at the bottom-left of the main text pane, or View > Show Meta Data or cmd-D.
To search by keyword you can simply type a keyword(s) into the search field.
As ever, you can copy the displayed clip using Edit > Copy, cmd-Alt-C or the toolbar button. Depending on your Preferences you can single-click or double-click the clip's name to copy the clip.