The Find dialog box allows searching the active document for a given string. You can show this window by clicking the Ctrl+F shorcut combination, or executing the Edit|Find menu command, from the main menu.
Though it looks like a modal dialog, it is a floating tool window, so you can switch between this window and the code editor without closing the dialog first.
Actually, both the Find and the Replace dialog boxes are two views of a same dialog box. As the above image suggest, you can change from the Find dialog to the Replace dialog by clicking the Replace button from the dialog's toolbar.
See also
The Integrated Development Environment
Project management
The Code Editor
Keyboard shortcuts
Replace dialog
Go To Line dialog