This is a house where one or more residents pays rent and the owner either lives in the house or elsewhere. Residents have their own bedrooms, but share some facilities, normally the kitchen and/or bathroom. A change of use of a house from occupation by a single household to an HMO with up to six residents (or vice versa) will not require planning permission unless the local council has made an order covering the whole or part of its area (known as an Article 4 Direction) that prevents such changes taking place without permission. You should check with your local planning authority.
If there will be more than six residents, planning permission will always be required. HMOs can raise concerns about matters such as noise, refuse storage and parking so you should check the policies of the council before getting your application prepared.
Regardless of whether planning permission is needed, most HMOs now require a licence from the Housing Department of the local council.