There is no difference. A three-phase 'star' connection is simply another name for a 'wye' connection. 'Star' seems to be the preferred name in Britain, whereas, 'wye' seems to be the preferred name...
A star or (wye) winding is a three phase four wire system. Three "hot legs" and a common neutral wire that connects all three phase coils together at one point where it is grounded to the system...
A delta forms where a river empties into a large body of water. It's usually something associated with medium to large rivers. It is generally the the build up a very fine particles of dirt (silt)...
There is no difference. 'Star' and 'wye' are alternative names for the same connection. Similarly, 'delta' and 'mesh' are alternative names for the same connection.