In 2011 effective altruists in Oxford had two main organizations: Giving What We Can and 80,000 Hours. They wanted to incorporate, and created the Centre for Effective Altruism as an umbrella organization to host them: 
Over time they started running other projects: conferences, supporting local groups, the EA forum, community health, etc. There was effectively a "CEA" community-focused organization within the "CEA" umbrella organization:
This was pretty confusing: when someone said "CEA" did they mean the organization focused on the EA community ("CEA runs EA Global") or the umbrella organization ("80k is part of CEA")? This got even more confusing as there started to be more organizations and projects:
In September 2022 the umbrella organization renamed itself to the Effective Ventures Foundation:
Unfortunately the announcement wasn't very clear about what specifically was changing, and a lot of people are still confused about when to say "CEA" and when to say "EV". Hopefully this history and the diagrams clear things up a bit!
[EDIT: changed 'EVF' to 'EV'; Shakeel says they prefer the latter.]
 This is also the origin of using effective altruism to refer to the movement.