The House of Lancaster is a term sometimes applied to one of the opposing factions involved in the Wars of the Roses, an intermittent civil war which affected England and Wales during the 15th century.
The House of Lancaster was so called because its claimants to the throne were all descended from John of Gaunt, Duke of Lancaster.
The opponents of the House of Lancaster were were the House of York. The rivalry between Lancaster and York, in the form of the counties of Lancashire and Yorkshire, has continued into the present day on a more friendly basis.