Who is running?
Umm, there are 650 seats total, each one can have tens of possible nominees.
But, if you're asking for the party leaders then they are as follows for the biggest parties and biggest Nationalist parties;
Conservative (aka Tories) (current co-government) - David Cameron (current Prime Minister)
Labour (Largest opposition party) - Ed Miliband
Liberal Democrats (current co-government, in minor position) - Nick Clegg (current Deputy Prime Minister)
UKIP [United Kingdom Independence Party] - Nigel Farage
Green Party - Natalie Bennet
Scottish Nationalist Party (only has representatives in Scottish constituencies)(current Scottish Parliament Government) - Nicola Sturgeon
Plaid Cymru (only has reps in Welsh constituencies)(currently 3rd largest party in Welsh Assembly, behind Labour(government) and Conservatives) - Leanne Wood
Whichever party gets the majority of seats (326) becomes the Government, and their leader becomes Prime Minister.
If there's a hung parliament (no party gains majority seats) then parties can form coalitions, whereby two parties add together their seats to create the majority. The largest party in the coalition has their leader as Prime Minister, while the next biggest is the Deputy Prime Minister. There can be more than 2 parties in a coalition.