According to the Wikipedia article about "Separation of powers" "[t]he phrase 'checks and balances' was ... coined by Montesquieu [I have not been able to verify this from other sources, and it is highly unlikely in my mind that Montesquieu might have coined a phrase in English... -CCH]. In a system of government with competing sovereigns (such as a multi-branch government or a federal system), 'checks' refers to the ability, right, and responsibility of each power to monitor the activities of the other(s); 'balances' refers to the ability of each entity to use its authority to limit the powers of the others, whether in general scope or in particular cases." (The same article, incidentally, includes a useful table of the reciprocal powers of the three branches of the US federal government, as an example of the general principle of the concept of the separation of powers.)
Whatever the origin of the expression, it is unquestionably true that Americans typically believe that the checks and balances embodied in the Constitution are essential to the protection of their individual rights and liberties, and are correspondingly proud of them, and even hold them to be a fundamental component of "American" ideology, even though, as the Cornell University Law Department's web-site points out, the doctrine of checks and balances is not explicitly mentioned anywhere in the Constitution, but is rather to be inferred from various provisions scattered through the first three articles. Some authors, such as Garry Wills in A Necessary Evil (1999), go so far as to regard American's veneration of the built-in inefficiency implied by the notion as a myth, but most legal scholars would probably agree with the interpretation to be found at the Cornell web-site (here).
The preponderance of powers accorded to the legislative strongly suggests that the authors of the Constitution had more confidence in that branch, in which power and authority are spread out among many people, who are answerable to the electorate, than in the executive, in which power is concentrated in a single individual, the President, or in the judicial branch, whose members are appointed for life, and are thus cannot be removed from office by elections. However, one of the general tendencies in American Constitutional history has been the growth of the power of the President,. Recent events tend to confirm this. (More here.)
In the abstract, then, the structure of checks and balances created in the Constitution seems to point unambiguously to a special confidence accorded by the Founders to the Congress.
When confronted with concrete reality, the abstract plan has not always worked exactly as might at first appear, however.
In particular, the impeachment power, that ultimate weapon of the Congress, is very rarely used, while the lesser powers of the other branches are often deployed.
For example, the Congress's impeachment power apparently outweighs the counterpart checks wielded by the judicial branch. In reality, however, no more than a handful of federal judges have been removed from office (and usually for unmistakable derelictions of duty, such as alcoholism).
On the other hand, the judicial branch, incarnated in the Supreme Court, has often used the power of judicial review to invalidate legislation passed by Congress. In other words, the statistics of use by the two branches of their respective checks show that the apparently less radical power (judicial review) has in fact been historically more influential than the more crushing power of impeachment.
The same can be said in the legislative branch's relations with the executive: here again, Congress is empowered by the Constitution remove from office a President who oversteps the bounds of the authority conferred upon him/(her) by the Constitution; the President, though s/he is entrusted with a (limited) veto power, cannot dissolve Congress, as the head of state can in many Parliamentary democracies.
However, in more than 215 years of American history, only two Presidents have been impeached, and neither was convicted of the impeachment charges.
The veto power, on the other hand, has been frequently invoked over the course of US history. The President has the power to veto legislation he deems incompatible with the Constitution or with his own political agenda, though of course the congress can override his veto by a two-thirds majority.
In the abstract, one might expect that the limits imposed by the Constitution are clear, and that a conscientious Congress would exercise / keep a vigilant eye on the activities of the President to insure that s/he respected the limits (the Constitutionally required report on the State of the Union provides an obvious mechanism of oversight). In reality, the impeachment power is in fact intimately linked with political considerations and political alignments and affinities.
Both of the impeachments that have taken place were politically motivated (The outcome of the impeachment trial of Andrew Johnson in 1868 was for many years assumed to have established the principle that impeachment charges would not be founded on purely political considerations, but it is hard to find other plausible motivations in the charges preferred against Bill Clinton.)
The Constitutional relations between Congress and the President today are manifestly colored by politics: President has in the absolute arrogated to himself more power than any other President (and has surrounded himself with councillors who strongly favor a nearly omnipotent Presidency), and has also defied demands by Congress to provide explanations and information necessary to the conduct of legislative and Constitutional deliberation. However, (and this has been especially true since the elections of 2004) the party affiliations and affinities of the President and the Congress have for all practical purposes immunized the President and his associates from effective oversight; it is unthinkable that the present Republican Congress might envisage impeachment of the current Republican President, though some of his actions could easily be viewed as impeachable. One example is the denial of habeas corpus to prisoners detained at Guantanamo and elsewhere as "illegal enemy combatants"; this measure is explicitly authorized by the Constitution in time of war, but the Constitution also provides that Congress has the exclusive power to declare war, and as has been mentioned elsewhere, Congress has issued no declaration of war since 1941.*
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* The "war on terror" has provided the Administration with opportunities to expand the powers of the Presidency to an unprecedented extent, under the cover of the secrecy allegedly justified by the demands of national security. Some "conspiracy theorists" have gone so far as to accuse the Administration of orchestrating the World Trade Center and Pentagon attacks of September 11, 2001, in order to create a socio-psychological atmosphere that would allow them to expand the powers of the President. This accusation beggars belief. On the other hand, since the President knows himself to be all but utterly exempt from Congressional disciplinary action, he has no scruples about pushing his executive authority beyond the limits that previous Presidents have respected. [back to text].