Your mum. The person who gave birth to you. You have a mum even if you are adopted. What did you expect.
The Bri'ish/Australian way of spelling mom.
Mum makes more sense than Mom. You don't go around saying mOm do you (unless your really American, in that case learn English properly dude) You say mUm.
When you need to swear down on something you say ‘mums’. Commonly used in London slang.
John: did you pull last night?
Jim: yeh man I did
John: Nah, I don’t believe you
Jim: Mums! I swear I did!