There are substantial differences within as well as between the systems of parliamentary democracy and presidential democracy. To take two specific examples as patterns, some of the differences and similarities between parliamentary democracy as practiced in the United Kingdom and presidential democracy as practiced in the United States are as follows.
1. The president is Commander-in-Chief, head of both state and government. The prime minister in a parliamentary democracy is supposed to be "first among equals," the leading minister who remains responsible to the monarch, who is head of state. While not all parliamentary systems have a monarch, most grew out of systems which used to have one.
2. Anyone can run for president, subject to certain conditions, and the president is directly elected by the people (though, in the United States, this occurs via the electoral college). In a parliamentary democracy, people vote for local candidates representing a particular party, and this party then chooses its leader. The leader of the party that can command a parliamentary majority (either through his/her own party or through a coalition) becomes prime minister.
3. Though a prime minister sits in the legislature (as does the entire cabinet), while the president does not, similar considerations affect the power of both leaders. A president may fail to command a majority in the House or the Senate, meaning that s/he can accomplish very little in terms of legislation. A prime minister who leads a coalition, or who has a very small parliamentary majority, is in a similarly weak position.
Parliamentary systems and presidential systems are both forms of representative government, which are run through elections (by which citizens select representatives to represent them). Additionally, they both tend to involve the use of political parties, which tend to play a critical role in shaping the practical realities of each political system.
However, while both presidential and parliamentary governments might be representative in nature, there are critical differences between the two systems. Perhaps most importantly, you should be careful not conflate the office of the prime minister and the president. A president is the head of the executive branch and is selected by its own electoral process. A prime minister, on the other hand, is the party leader for whichever party holds power in the legislature at any time. When elections are held, a new prime minister is selected if a new party gains political ascendancy within parliament. From that perspective, the clear distinction between the executive and legislative branches, which exists and defines presidential democracy, does not exist under a parliamentary system.
There are similarities and differences between a parliamentary democracy and a presidential democracy. One similarity is that in both forms of government, the people elect their representatives. This gives the people significant power since they are able to decide who will represent them.
There are differences between the two forms of democracy. The United States has a presidential democracy, while the United Kingdom has a parliamentary democracy. In a presidential democracy, separation of powers is important. There are specific jobs that each branch of government has. This means that executive power doesn’t exist in the legislative branch. Also, the President is elected separately from the members of the legislative branch. In a parliamentary democracy, there is no election for the chief executive. The majority party in the legislative branch will choose that person. This makes it easier to pass laws because there is no threat of a presidential veto in a parliamentary democracy. Because presidential democracies generally have a system of checks and balances, this may limit what each branch of government can do.
The major similarity between a presidential democracy and a parliamentary democracy is that both of these types of governments are democracies. This is a fundamental similarity that is, perhaps, more important than any differences. Both of these forms of democracy allow people to vote for their leaders. This makes them superior to other forms of government that do not allow their people to enjoy popular sovereignty.
However, there are important differences between the two forms of democracy. First, a parliamentary democracy is less likely to encounter gridlock than a presidential democracy is. In a parliamentary system, there cannot be a situation in which the chief executive is from one party while the legislature is (in whole or in part) controlled by a different party. This makes the parliamentary system more efficient. Second, a presidential system allows for separation of powers where a parliamentary system does not. In a presidential system, there is a clear distinction between the legislative and the executive branches of government.
Thus, these two types of government are similar in the most important way. However, a parliamentary system is more likely to be efficient while a presidential system allows for separation of powers and, thereby, greater protection of people’s rights.