Inverters work by taking in battery power and producing normal mains power. In other words they allow you to connect ordinary mains appliances to your car battery. There are a few checks you need to make first though, unfortunately it is not always as easy as that.
How much power does the mains appliance draw? You need to match the power of the appliance to the power of the inverter. Most mains appliances have a sticker on them somewhere giving their power rating, but as a guideline small devices like MP3 players or cellphone chargers are very low; laptops are around 100W but heating devices like kettles or microwaves start at 1000W.
Another technical detail is the run time of the battery, if you are powering a kettle from a normal 12V car battery it will be flat in around 30 - 45 minutes. In these circumstances you need to make sure you are not going to run your car battery so flat you can't start in the morning. You should rather be looking at a deep cycle battery with some means of recharging it. Talk to us, we have a few ideas.