Parties and their stances:
Conservative and Unionist Party: centre-right, liberal conservative. In practice, they're pretty socially liberal, what with their (hestitant) support for gay marriage and three parent babies. They are mildly Eurosceptic. They are the bigger party in the current coalition governement and they're led by David Cameron.
Labour Party: centre-left, social democratic. After the 2008 crisis and the devastating election of 2010, they were relegated to opposition benches and blaming them for economic mismanagement has become the national pasttime. They are lead by Ed Miliband, who defeated his brother, David in leadership election, which was terribly unfortunate for Labour, because 1) Ed was terribly unpopular and 2) David actually won the popular vote, but was defeated by the trade union vote, which contributed to public perception of Labour as a party subervient to trade unions out to wreck the economy. In debates, though, he performed decently, and the public opinion has largely swung in his favour, so most people no longer cringe at the thought of him becoming PM.
Liberal Democrats: center, liberal, social liberal. Currently the small party in the government. They were the third party in 2010, getting 23% of the vote, but because of the FPTP system used in Britain, they get less than 10% of seat (in other words, for every 10 Labour voters, there were 8 Liberal Democrat voters, but for every 10 MPs Labour got, Lib Dems only got 2), which is why they campaigned to changed it to AV, which is only marginally less shitty, and it failed in the referendum. They are expected to lose a lot of seats, because they half of their electorate was either disgruntled Labour voters (whom they angered by going to a coalition with a Tories) or students (whom they put off by promising to vote agianst any raise in tuition fees and then voting to raise them; it was a dumb policy to begin with, the current system of student loans is as fair as it gets, but since people like getting free things, free meaning funded by the taxpayers, students voted in droves for whoever promised to abolish the tuitions fees).
Green Party of England and Wales: left-wing, green. They combine environmentalism with radical left-wing policies. Their leader is pathetically incompetent Natalie Bennett.
UKIP: right-wing, conservative. Previously a fringe party, UKIP are expeced to get a lot of votes this election. They are very Eurosceptic and anti-immigrantion filling the void that was created when the Conservatives decided to stop being conservative. They won the European Parliament election and were polling pretty high since. Their charismatic leader, Nigel Farage generally pretends to be an ordinary bloke, that only supports common sense policies and fight the corrupt and greedy political establishment. They attract a lot of working class vote, polling at about 13%, but, thanks to wonderful FPTP, are expected to win less than 1% of the seats.
RESPECT Party - far-left, socialist. Them winning a seat must be a fault in the algorithm (another one is SNP winning Orkney and Shetland, the archipelago in the far north, which is an extremely safe Liberal Democrat seat). They are basically the worst of the loony left.
Regional parties:
SNP: left-wing, Scottish nationalist. Previously a small party, after the independence referendum, they're expected to win 90% of the Scottish seats (with 45% of the vote, hurray FPTP!). They're genrally seen as left of Labour and their rhetoric is pretty radically left-wing, but their governance of Scotland really isn't. When not going on about Scottish independence, they're mostly concerned with scrapping Trident, the British nuclear deterrent, stationed in Scotland, something nobody cared about but they made it a top issue to whip up anti-English hostility, which they benefit from. Some people say thay the same thing that happened to Ireland will happen to Scotland, leading to eventual independence, which quite frankly would be a shame, given how much has Scotland and England together been through.
Plaid Cymru: left-wing, Welsh nationalist. Welsh SNP minus the popularity.
DUP - right-wing, conservative. Ulster nationalists. Very Conservative, hate the Irish.
Sinn Fein - left-wing, Irish nationalist. They are absentionist - they don't sit in House of Commons. Because of them, and the Speaker, who doesn't vote, only 323 seats are needed to get a majority. One day, the Conservatives will get 323 seats and it will be the day that Sinn Fein abandon their absentionism. Until this day, they will ensure that the vote put for them in the ballot box is worth no more than a vote put in the rubbish bin.
SDLP: center-left, social democratic. In practice a Northern Irish branch of Labour.
Alliance - centre, liberal. Previously a fringe party, Alliance achived fame after their candidate deprived the leader of DUP of his seat. It would be a pity if they got no seats this time around.
Sylivia Hermon - centre-right.