The graph above (from The Economist) shows the following:
The solid circle shows the how the actual number of votes cast was split. The solid coloured blocks within the solid circle show the number of seats won (there are 650 seats in the UK House of Commons).
The empty coloured blocks show the number of seats that the party would have won had it been directly proportional to the number of votes cast.
The solid coloured blocks outside the circle show the extra seats gained due to the first-past-the-post system.
Both major parties, Labour and Conservative, have many more seats than they have as a percentage of votes cast.
It is the reverse for the smaller parties, except for the SNP (Scottish National Party). SNP is different because its votes were geographically concentrated, allowing to win extra seats.
UKIP won about as many votes (12.7%) as SNP (4.7)% and the Liberal Democrats (7.9%) together yet it has only one seat. By contrast, SNP, with only 4.7% of the vote, has 56 seats.
Note 1: the fractions are just what I could find..not all sources agree to the third digit: I cannot find the formal raw data (I looked for about 2 hours).
The chart below (from The Telegraph) shows the seats and how they have changed since the last election:
Note 2: The top chart from The Economist is available for several previous elections (it is an interactive display)