Contrary to the above suggestions the term "your mum" is not actually officially an insult. In fact, in many situations it can be seen as a very honourable compliment. For example, if someone calls your mum a "hottie", you can revel in the knowledge that you were first out of all your friends to be in her (provided none of your friends have entered her previous to your birth, might be best to check first). Additionally, your mum might be called "loose" and a "fucking slag" which are street terms for being a good listener.
However, there are certainly usages which can be deemed offensive, as we can see below in the examples. In fact, the British government in the "Your Mum" parliamentary act of 1923 split the term into 5 classes. Use of a level 5 can lead to unlimited fines and/or a sentence of 20 years 'tough love' in the slammer. Community service for a level 4 usage is compulsory, servicing 40+ year old mums to 'keep them fresh'.
The term was first coined by early Christians after the Bible story in Bernard 3:19 in which Jesus is still a teen and beginning his forages into carpentry and the bush. In the account he is bullied by a gang of locals nicknamed 'The Disciplz', who continually shout abuse and throw pasties at his knees . Eventually, Jesus tires of the pastry-related insults and hits back with his best friend Foreskin, cornering the Disciplz and threatening them with a badger. His tormentors are reluctant to repent until Jesus speaks the most momentous words that possibly the whole world has ever heard...
"Your mothers are all very much like the Romans. They are strong, valiant and innovatice, and they all wear skirts that show their willies."
In the story the Disciplz immediately fall to their knees and pledge allegiance to Jesus, remaining by his side for 20 more years, even when they needed the loo.
Inevitably the usage grew from that point but the term has been somewhat tainted by popular culture and Richard and Judy. We can only pray that those who use it correctly shall be rewarded with the "Second Coming (Of Your Mum)"...
Level 1 - Much favoured usage in the Western world
'Your mum has a massive cock and I am going to stick it in my mouth and inflate her already huge face'
Level 2 - A passable compliment
'I would love to take your mum out sometime and penetrate her, if that is ok with you'
Level 3 - Risky, preferably used behind closed doors
'Your mum likes chocolates because they are the same colour as my poo'
Level 4 - Offensive and nasty use - would make me cry
'Your mum has the arse of an elf.'
Level 5 - You must be a mental
'Your mum is a M*ll*rd Duck'