PrefixQuery, as the name implies, matches documents with terms starting with a specified prefix. WildcardQuery allows you to use wildcard characters for wildcard matching.
A PrefixQuery is somewhat similar to a WildcardQuery in which there is only one wildcard character at the end of a search string. When doing a wildcard search in QueryParser, it would return either a PrefixQuery or WildcardQuery, depending on the wildcard character's location. PrefixQuery is simpler and more efficient than WildcardQuery, so it's preferable to use PrefixQuery whenever possible. That's exactly what QueryParser does.
Here is a code snippet to demonstrate both Query types:
PrefixQuery query = new PrefixQuery(new Term("content", "hum"));
WildcardQuery query2 = new WildcardQuery(new Term("content", "*um*"));