Are you a contractor? If you work for someone else, it is important to know whether you are working for that person in an employed capacity or in a self-employed capacity as an independent contractor.
Your employment status will determine the charge to tax on income from your employment or self-employment. It will also determine the class of NICs, which are to be paid.
Umbrella Companies employ you for tax purposes and complete central returns for all their contractors. It's less hassle and more flexible for you but take home pay is typically less than that the limited company option.
The best option for you depends on your plans, 'one off' contract or 'contractor for life'?.