The British MPs have often described congress lobby fodder but defy the party whip. This is of course to the house committees where select committee have limited power and resources however in the senate and House of reps the standing committees are fully professional staffed and have the power or death over legislation. Congress is split into two houses the senate and the house of reps (HOR) both of these have individual powers have they also have joint powers. Meaning that due to check and balances no one part of congress can dictate what happens. There are many ways in which congress can be judged to be effective or not firstly powers, secondly oversight, thirdly legislation and finally gridlock.
Of course there are huge power differences between congress and parliament. In parliament powers are constraint by royal prerogative, this therefore means the prime minister acts on behalf of the queen. Although MPs could vote before British troops were deployed to Iraq the PM had the right to take the country to war without consent from parliament. This is of course in contrast to the US. Were specific powers are assigned in the constitution. Although the president is commander in chief you would of thought that he would be able to declare war. However this is not the case. Only congress can declare war. This was put in place after the Vietnam War when congress but the wars powers act in to action. Not only this but congress has the power of the purse.
Of course the senate has joint powers with the HOR. If the president decides to veto a bill for whatever reason congress can overturn it. However for the to be achieved. There needs to be two thirds majority in each house. The treaties which the president agrees with other countries need to be ratified by the senate. The appointments made by prime ministers also have to be confirmed by the senate.
Oversight is where congress is where they scrutinise and check the executive branch,