The "Find Previous" and "Find Next" buttons will search backwards or forwards through the text, trying to find whatever has been typed into the "Find" field. You can also use the <Return> key after typing in the "Find" field to find the next occurrence (and <Escape> to return to editing the text without having to use the mouse).
The "Replace" button searches for the text that has been entered in the "Find" field and replaces it with the contents of the "Replace" field. "Replace All" searches through the whole text, replacing every occurrence listed in the "Find" field with that listed in the "Replace" field. You are given the option to cancel this before it starts, because it may take a long time to undo every replacement if you make a mistake. The message area at the bottom left of the window tells you when the operation is finished and how many words were replaced.
All Replace operations ensure that initial capital letters are preserved, so if for example you want to replace all instances of "large" with "big" and a match of "Large" is found (with a capital 'L') the replacement word will be "Big" rather than "big".
The "Case Sensitive" and "Whole Words Only" checkboxes affect all Find operations. "Case Sensitive" means that matches are only found if the capital and small letters match exactly (e.g. if "Ail" is searched for and "Case Sensitive" is on, text such as "ail" and "AiL" will not be found). "Whole Words Only" means that matches are only found if they are distinct words in their own right and not part of another word (e.g. if "ail" is searched for and "Whole Words" is on, only the word "ail" will be found; if "Whole Words" is off, "ail" will be found inside words such as "ailment" and "rail").
The "Count Instances" button takes the word typed into the "Count Instances:" field and counts how many times it occurs in the current file. The "Case Sensitive" and "Whole Words Only" checkboxes work in the same way as described above.