The '!' represents an Exclusion Location, ie a specific path, whereas the '-' represents an Exclusion Filter, ie text that is matched against the path.
For example, to exclude the System32 folder you could use:
If there was a problem with your exclusion location, e.g. the System32 folder didn't exist, the program would warn you.
Alternatively, you could use:
This would match against all the paths being searched and would prevent ANY path with System32 being searched. However, you wouldn't get any warnings if the System32 folder didn't exist.
For more information on the Look In field please see: