A proportional representation electoral system differs from a first-past-the-post system primarily in how votes are translated into seats in the legislature. In a proportional representation system, parties gain seats in proportion to the percentage of votes they receive, allowing for a more accurate reflection of the electorate's preferences. In contrast, a first-past-the-post system awards a single seat to the candidate with the most votes in a district, often leading to a winner-takes-all outcome. This can result in less diverse representation and disproportionate outcomes, whereas proportional systems tend to facilitate multi-party participation.