A custom is like a group filter, but (usually) just for two people and a way to interact without the whole community being able to read. Some people have more than that in a custom and you can have multiple customs. But when someone adds you to a custom, you can use it for sceneing, chatting through comments, or whatever. It's like having the privacy of being on AIM without actually being on it. What you do on the custom is up to you and your partner, but you can do both!