Window identifiers are integers, and are used to uniquely determine window identity in the event system (though you can use it for other purposes). In fact, identifiers do not need to be unique across your entire application just so long as they are unique within a particular context you're interested in, such as a frame and its children. You may use the wxID_OK identifier, for example, on any number of dialogs so long as you don't have several within the same dialog.
If you pass -1 to a window constructor, an identifier will be generated for you, but beware: if things don't respond in the way they should, it could be because of an id conflict. It is safer to supply window ids at all times. Automatic generation of identifiers starts at 1 so may well conflict with your own identifiers.
The following standard identifiers are supplied. You can use wxID_HIGHEST to determine the number above which it is safe to define your own identifiers. Or, you can use identifiers below wxID_LOWEST.
#define wxID_LOWEST 4999 #define wxID_OPEN 5000 #define wxID_CLOSE 5001 #define wxID_NEW 5002 #define wxID_SAVE 5003 #define wxID_SAVEAS 5004 #define wxID_REVERT 5005 #define wxID_EXIT 5006 #define wxID_UNDO 5007 #define wxID_REDO 5008 #define wxID_HELP 5009 #define wxID_PRINT 5010 #define wxID_PRINT_SETUP 5011 #define wxID_PREVIEW 5012 #define wxID_ABOUT 5013 #define wxID_HELP_CONTENTS 5014 #define wxID_HELP_COMMANDS 5015 #define wxID_HELP_PROCEDURES 5016 #define wxID_HELP_CONTEXT 5017 #define wxID_CUT 5030 #define wxID_COPY 5031 #define wxID_PASTE 5032 #define wxID_CLEAR 5033 #define wxID_FIND 5034 #define wxID_DUPLICATE 5035 #define wxID_SELECTALL 5036 #define wxID_FILE1 5050 #define wxID_FILE2 5051 #define wxID_FILE3 5052 #define wxID_FILE4 5053 #define wxID_FILE5 5054 #define wxID_FILE6 5055 #define wxID_FILE7 5056 #define wxID_FILE8 5057 #define wxID_FILE9 5058 #define wxID_OK 5100 #define wxID_CANCEL 5101 #define wxID_APPLY 5102 #define wxID_YES 5103 #define wxID_NO 5104 #define wxID_STATIC 5105 #define wxID_HIGHEST 5999