The effect of a system based on plurality voting spread over many separate districts is that the larger parties, and parties with more geographically concentrated support, gain a disproportionately large share of seats, while smaller parties with more evenly distributed support gain a disproportionately small share. This is because in doing this they win many seats and don't 'waste' many votes in other areas. As voting patterns are similar in about two-thirds of the districts, it is more likely that a single party will hold a majority of legislative seats under FPTP than happens in a proportional system, and under FPTP it is rare to elect a majority government that actually has the support of a majority of voters. Because FPTP permits many wasted votes, an election under FPTP is more easily gerrymandered. Through gerrymandering, electoral areas are designed deliberately to unfairly increase the number of seats won by one party by redrawing the map such that one party has a small number of districts in which it has an overwhelming majority of votes (whether due to policy, demographics which tend to favour one party, or other reasons), and many districts where it is at a smaller disadvantage.
