CCEA and WJEC exclusively serve Northern Ireland and Wales respectively.
Because the reforms in England conflict with the Welsh education system, WJEC now offers qualifications in England under the name Eduqas.
As to differences between them, there isn't much. The content of the specifications is prescribed by the DfE and there is little difference between the exams offered by the different boards. The main differences are stylistic, with different formatted papers, though topics might differ. For subjects with different options, eg. history, your new school might pick different options, so you'll have to catch up.
What will be most helpful to you would probably be to compare the specification used by your old school with that of your new one.