An umbrella company offers many of the advantages of running your own company but without the hassle. Like an agency, they employ contractors to work for other companies. However, the contractors are usually expected to find and manage their own contracts.
The umbrella usually represents a collection of contractors in similar fields. They usually provide payroll, invoicing and can provide additional pay through dividends. It's also possible to get discounts on insurances, legal, and accounting costs since an umbrella can bulk buy.
Some also offer benefits packages that are similar to agencies. The packages offered by different umbrella companies run from a simple payroll through to a complete service covering holiday, insurance, pension, sickness, and accounts.
For the purposes of tax, you are an employee of the umbrella company. It will deduct your tax and NI contributions through PAYE.