Ok, here are some basics.

First, and importantly, the issue of reservations is conceptually distinct from the self-executing issue. Let me explain the latter first.

Legal systems are commonly classified as either “monistic” or “dualistic.” Under a monistic system, international law and domestic law are both part of the same legal order. Thus, if a country with a monistic legal system ratifies a treaty, the rules of law contained in that treaty become rules of law in that country in the same way that rules derived from duly-enacted legislation would. (Note that this is very much a simplification of reality. I’m leaving out some important points, but they’re not necessary for generally grasping the difference between monism and dualism.) Under a dualistic system, international law and domestic law form two distinct legal systems. If a country with a dualistic legal system ratifies a treaty, the rules of law contained in that treaty will bind the country on the international level, but the rules won’t have any effect in the national legal system. In order for the rules to have effect in the national legal system, the legislature (or whomever, depending on the national constitution) will have to pass legislation that implements the treaty. (Note again that this is a simplification, but it’s useful enough as a rough model. In reality, most countries have some form of a dualistic system, though it’s not as clean and neat as this division makes it appear to be.)

Article VI of the Constitution provides in part that “all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land.” That sounds like we have a monistic system, right? Alas, we do not.

Federal courts in the United States have long recognized that some treaty provisions are “self-executing” and some are not. U.S. Supreme Court Chief Justice John Marshall discussed the distinction in an 1828 case, Foster v. Neilson: “Our constitution declares a treaty to be the law of the land. It is, consequently, to be regarded in courts of justice as equivalent to an act of the legislature, wherever it operates of itself, without the aid of any legislative provision. But when the terms of the stipulation import a contract, when either of the parties engages to perform a particular act, the treaty addresses itself to the political, not the judicial department; and the legislature must execute the contract, before it can become a rule for the court.” (27 U.S. (2 Pet.) 253, 314).

A single treaty can contain both self-executing and non-self-executing provisions. If a treaty provision is self-executing, no congressional legislation implementing that provision is necessary. If a treaty provision is not self-executing, Congress will have to implement the provision through legislation in order for the provision to be binding in domestic law. The distinction is important because where a treaty provision is self-executing, judges will have the power to apply the provision in relevant cases (again, simplifying). Judges do not have power to apply a non-self-executing treaty provision, where the provision has not been implemented.

The result is that where a treaty provision has been implemented (through legislation), the judge will apply the rules found in the legislation, not the treaty itself. Therefore, if there are any differences between the treaty and the implementing legislation, the version found in the implementing legislation will win (at least in domestic courts).

So how do we tell if a treaty provision is self-executing or not? The general rule is that we look to the intent of the parties to the treaty. Did the parties intend for the provision to be self-executing or not?

Now we come to the issue of reservations. Article 2(1)(d) of the Vienna Convention on the Law of Treaties (VCLT) defines a “reservation” as “a unilateral statement, however phrased or named, made by a State, when signing, ratifying, accepting, approving or acceding to a treaty, whereby it purports to exclude or to modify the legal effect of certain provisions of the treaty in their application to that State.” Article 19 of the VCLT allows states to make reservations, unless the reservation is specifically prohibited by the treaty itself, the treaty only allows for specific reservations and the reservation in question is not one of those specified, or the reservation is “incompatible with the object and purpose of the treaty.”

There are also things called “understandings” and “declarations.” The precise differences between reservations, understandings, and declarations is a debated issue, and we need not go into it here.

Now, as we know, Art. II, section 2 of the U.S. Constitution gives the President “power, by and with the advice and consent of the Senate, to make treaties, provided two thirds of the Senators present concur.” If the Senate only grants its consent on certain conditions, the President must act accordingly. In other words, the President can’t just ignore the conditions on which the Senate gave its consent. If he did, we wouldn’t be able to say that the Senate consented, and therefore we wouldn’t be able to say that the treaty was made “under the authority of the United States” as prescribed in Art. VI of the Constitution.

The Senate will sometimes give its advice and consent only “subject to” certain reservations, understandings, and declarations. Take the Convention Against Torture. The Senate gave its advice and consent subject to 2 reservations, 5 understandings, and 2 declarations. The first declaration was that Articles 1 through 16 of the CAT “are not self-executing.” Thus, part of our intent in ratifying the Convention was that the specified articles were not to be self-executing. (See Restatement (Third) of Foreign Relations Law § 111(4)(b) (1987) (international agreement non-self-executing where Senate gives consent on condition that implementing legislation required)).

There are lots of issues, and a lot of debate, about much of what I’ve said above; also, the issues are more complex than what I’ve said above, but I think I’ve adequately given you the basic ideas. I should mention, in closing, that states are under an international obligation to adjust their domestic law to be consistent with and give effect to a treaty. Sometimes that won’t require any changes (where domestic law adequately gives effect to the treaty already). Where domestic law does require changes, and the state does not change its domestic law, the state is breaching its obligation. Of course, there’s usually no way to enforce that obligation, or to hold a state accountable for such a breach, beyond publicly calling them out on it.

I hope that helps. I can point you to some articles and whatnot, but I’m not sure of what kind of access you have to law reviews, etc. Let me know if you want those references, anyway.

Okay, rain has forced me back inside, and I’ve found an answer to my question at the end of the comment. It’s not a pretty picture; most countries’ reservations to the CAT are waaay less sweeping than ours.