In a parliamentary system, the legislative branch is supreme and executive power lies with a majority of members of parliament who select a Prime Minister and departmental ministers. This may be the leader of the majority party or, as is more common in Australia and some other Commonwealth systems, it is a member of the cabinet who is not a political party leader. Parliamentary systems vary in the degree to which executive power is vested in an independent head of state (a hereditary monarch in a constitutional monarchy, for example) and whether or not the legislative branch is able to declare laws unconstitutional.
Most parliamentary systems have either a plurality or some form of proportional representation for electing members to parliament. Plurality tends to have single-member constituencies, and the result is that the candidate with the most votes wins a seat. In the UK, for example, there are 650 constituencies and each year people vote for one of their chosen candidates to be their MP. If the winner of a seat is not from the same party as the voter, this is called a hung parliament and the governing party must seek coalition partners to form a government.
In multimember systems like SNTV or the PR-STV, seats are allocated to parties according to a formula based on their share of the overall vote. A party’s seats are first apportioned to their candidates in each electoral district, and any leftovers are awarded using the Sainte-Lague formula to ensure that the number of seats won by a party closely matches its share of the overall vote.