Who are the G8 countries?

The G8, the Group of 8 is made up of Canada, France, Germany, Italy, Japan, Russia, the USA and the UK. 

The European Union is represented by the President of the European Commission, and the President of the European Council.

Russia was suspended from the membership of G8 in 2014 that effectively dissolved G8 and became G7 (group of 7).

Further details on Group of Eight (G8) are available on the given link.

Further Readings-

1. Group of Seven (G7)
2. European Union