In other words, the legislature is elected with one seat per constituency, one vote per voter, and whoever gets the most votes in the constituency wins, even if that's not a majority of the votes cast.
How does this work in practice? Well, at the
2005 general election:
Labour got 35.3% of the votes, and 55.2% of the seats
Conservatives got 32.3% of the votes, and 30.7% of the seats
Lib Dems got 22.1% of the votes, and 9.6% of the seats
In 2005, out of 650 seats, about 220 were won by a candidate with more than 50% of the vote, about 210 with between 45 and 50%, 160 between 40 and 45%, and 60 with less than 40% of the vote in their constituency.
Now, there are complicating factors - Northern Ireland has an entirely different set of parties standing, there are significant nationalist parties in Scotland and Wales that turn those nations into a 4 way race (ie all 4 parties have a realistic chance of winning at least one seat somewhere in the nation), and it's difficult to tell how much voters will not bother voting if they feel sure their prefered party is bound to win their seat.
What the FPTP system does encourage, in a situation with 3 or more viable parties, is tactical voting - if you have a prefered party, but think they can't win your seat, and much prefer one of the other parties to a third one, then you vote for your second choice, to make sure the real bastard doesn't get in. Labour has suddenly started talking about introducing the Alternative Vote system (known as Instant Runoff in the US) recently - either as a sop to the Lib Dems (who would benefit from it) or because they've worked out it'd hurt the Tories more than it'd hurt Labour.
Anyway, the effect of FPTP is for the more popular parties to get a disproportinate share of the seats. Many in Britain actually like this, because they fear coalition government, and some have a fetish for 'strong government'. At the moment, the distribution of seats and/or the likelihood of voting in safe seats appears to mean that the Tories will have to win about 5% more of the national vote than Labour to end up with the same number of MPs (eg Tories tend to win rural seats by a larger margin than Labour wins urban ones). Which may be a desirable effect, subjectively, but it's not exactly a perfect democracy.