Contents insurance is cover that can be taken as an addition to your buildings insurance or as a free standing cover in the cases were you have no control over were the building is covered such as a leasehold flat.

Cover is mainly based on the same risks as buildings insurance but will also include theft due to burglary and additional cover such as accidental damage, mentioned later.
Just as with Building Insurance you should ensure that cover is adequate to re-instate all the belongings in you home and to do this is it always a good idea to write a list of what is in each room of your house and the cost to replace each item.

You should also note things like carpets and curtains as these may not be covered un the buildings policy.

Typically a room can have £3,000 to £5,000 in contents value in it, may be more, so it is an important exercise.
As mentioned earlier there are additional benefits you can have they are as follows :- 

accidental damage cover. This is when a claim can be made if something in the home is damaged accidentally such as one of the kids spills a drink over the video. You would be able to make a claim to replace that item.

Legal Protection. This is when you have cover for legal costs in the event that someone takes legal action against you for such things as your dog bites someone and they try and sue you. This is very valuable cover as long as it is correctly set up so you should check what levels and type you have.

Away From Home. Mainly designed for personal belongings such as cameras jewelry credit cards etc. The cover is for when these things are taken out of the home and are lost or stolen. Like all the intricacies of your insurance you should check the policy wording to ensure you have what you want from the policy.