The start of the Wikipedia article defines it and explains what crimes it is commonly applied to.

https://en.wikipedia.org/wiki/Capital_punishment

Some interesting Guardian articles about Capital Punishment practice, including racial bias in sentencing.

https://www.theguardian.com/world/capital-punishment

This BBC ethics guide provides useful statistics on the use of Capital Punishment around the world.

http://www.bbc.co.uk/ethics/capitalpunishment/intro.shtml

Another BBC page with world executions in 2017 and contrasting figures on states still practicing it from the UN and Amnesty International.

http://www.bbc.co.uk/ethics/capitalpunishment/intro.shtml

Capital Punishment UK.  The history of the death penalty in Britain.

https://www.capitalpunishmentuk.org/

BBC ethics again. For and against arguments.

http://www.bbc.co.uk/ethics/capitalpunishment/

Pros and Cons on Debatewise.org

https://debatewise.org/debates/2588-capital-punishment/