Bicameral systems are legislative branches in which there are two separate legislative bodies. These two legislative bodies are generally different sizes and have different powers, even though they represent the same people. As with most political systems, bicameral legislatures have a variety of advantages and disadvantages.
A bicameral system divides power, which keeps it from being too concentrated in any one group's hands. In the United States, for example, the House of Representatives starts appropriations bills, while the Senate confirms presidential appointments and approves foreign policy decisions. By spreading power like this, the entire system is kept in check, as it is harder to influence two houses than it is to influence one house.
Broad vs. Focused
In most bicameral systems, one of the houses has substantially fewer people than the other, although each represents the same population. This means that the house with more people has fewer people per representative. This lets each of the lawmakers from the larger house (the House of Representatives, in the United States) represent a smaller area and focus on local issues, while the representatives from the larger house can focus on bigger-picture issues.
A bicameral system is, by definition, 100 per cent more complex than a unicameral system. Two houses mean two sets of rules, two sets of election procedures and two different kinds of representatives. While this theoretically represents people better, it can also do the opposite. Having two houses means that politics becomes more of an exclusive club; people who understand the system better have better access to influence, and the more complex the system, the fewer the people who understand it.
You can argue that a bicameral system is less representative than a unicameral system. This is because power that is spread out is also diluted. If, for example, someone supports a House candidate's foreign policy stance but cannot find a Senate candidate with the same stance, that voter is essentially out of luck -- his House member, if elected, will have a limited voice in foreign policy. The converse goes for appropriations.