Paul has summed it up pretty nicely there, basically if you register, you must charge VAT on every job. That is 17.5%
(So for a job costing £100, you charge £117.50)
But you don't get to keep that extra, you keep account of it and every three months you send it to HM Customs and Excise.
If you are charged VAT, for example on petrol, or equipment, you can deduct that amount from what you have to pay over.
If your work is all for VAT registered companies, they wont care less if you charge VAT, because they'll simply claim it back on their own VAT return. However, you must also charge it to domestic customers, which means their price goes up by 17.5% for nothing.
Once your turnover reaches £54,000 per year, you MUST register like it or not. There are severe penalties for not doing so.
Silly