Members of the European Parliament organise themselves into Political groups of the European Parliament, of which there are seven:
(In addition, some MEPs are not a member of a group; these are called Non-inscrits.)
All of these groups, with the exception of the European People's Party, contain MEPs from the UK.
European Parliament Groups should not be confused with European political parties (which are often informally called Europarties).
A European Parliament group exists within the European Parliament. Its function is to group like-minded MEPs together. European Parliament groups are not allowed to campaign during the European elections since this is the exclusive responsibility of the Europarties.
Europarties do campaign in European elections, since this is their function. They receive money from the EU to do so.
Even though European Parliament Groups and Europarties are separate, they are sometimes very closely related. For example, the S&D group has 184 MEPs, the vast majority of whom belong to parties that are affliated with the Party of European Socialists, a Europarty.