The Search command (Alt+F) searches the current file for a search pattern. The search can be forward or backwards. In addition, the search can be batch style, where the search output results are placed in a Search Results window. See: Search.
The Search Forward (F4) command repeats the last search, allowing you to advance through the file, finding each occurrence of a pattern.
The Search Forward for Selection command searches for the next occurrence of the first word in the current selection.
The Incremental Search command (F12) finds matches as you type the characters. See: Incremental Search.
The File Search Bar is used to search within the current file. To show it, select View > File Search Bar. Alternatively, you can use Activate Search Bar (Alt+Shift+F) to activate it so you can start typing into it.
To set options, click the Options button. See: File Search Bar Options.