At base, a congress and a parliament seem to be very similar things the legislative branches of the majority of the world's democracies which are filled with popularly elected men and women who come together to decide the legislative issues of their home nation. However, despite the initial impression of sameness, the two are very different in two key areas: how one is elected to the body and what one does once elected. The difference can begin to be seen in the origins of the words.
Congress is based on an ancient Latin word meaning "a coming together," which is precisely what happens in a congress representatives from all the parts of a nation coming together to discuss matters of state. Parliament, however, has its roots in a French word meaning "To talk," and there certainly is a great deal of talking going on in any parliament. In fact, this simple linguistic difference leads one to the chief difference in the election process.
A congress is based upon primary elections in which the general populace selects their candidates more based upon their personalities and individual plans for office, while in a parliament, the delegates are chosen to run by their own political parties based almost entirely on their willingness to follow party standards. While party certainly comes into play in any congress, the individual really does mean more in elections to that body. And it is that sense of individuality that leads to the second main difference between the two.
In a parliamentary government, the Prime Minister and his cabinet (The leaders of the nation) are chosen from the majority party in the country's parliament. Thus, if the members of that party begin voting against their party's ideals in the body, the entire government can come apart, forcing an election of a new Prime Minister. With this in place, many parties very carefully restrict the freedoms of their delegates to ensure the safety of their Prime Ministers.
In a congress, however, the executive branch is entirely separate from the legislative, allowing the members to vote freely based on the wishes of their constituents and own consciences, with no fear of doing permanent harm to government itself. Their increased individual power leads to many interesting phenomena, not the least of which being that they receive a much larger salary than parliamentary delegates individual congressmen are simply more important. And thus do the two differ from one another in many ways, making each a great deal more than a simple collection of delegates arguing about issues of state.