(Preliminary remark: Your "question" contains at least six questions, half of which are so broad that you might be better served by studying some introductory material on international law.)
(Re ¶1) International law is "the legal system governing the relationships between countries; more modernly, the law of international relations, embracing not only countries but also such participants as international organizations and individuals (such as those who invoke their human rights or commit war crimes)." Black's Law Dictionary (11th edn, Thomson Reuters 2019). "International law" does not refer to some big book of statutes (if that is what you mean), it's just the totality of all the treaties, agreements, regulations, etc. that exist.
A treaty is "an agreement formally signed, ratified, or adhered to between two countries or sovereigns; an international agreement concluded between two or more states in written form and governed by international law". Id. Similarly, the Vienna Convention defines "treaty" as "an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation". Vienna Convention on the Law of Treaties 1969, art 2(a). As one can infer from that, agreement is a broader term (encompassing informal agreements, for instance). Of course, the fact that something is called an agreement does not mean it is not, technically, also a treaty.
(Re ¶¶2&3) The consequences depend on the particular treaty and on what the parties have agreed on. The WTO, for instance, operates a sophisticated dispute resolution mechanism (see the Wikipedia article). If a particular treaty contains no such stipulations, it us up to the parties to decide what course of action they intend to pursue if they believe that another party does not fulfill its treaty obligations. States are generally bound by art 2(3) of the UN Charter, which provides that "all Members shall settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered". In resolving disputes, they must furthermore obey art 33(1): "The parties to any dispute, the continuance of which is likely to endanger the maintenance of international peace and security, shall, first of all, seek a solution by negotiation, enquiry, mediation, conciliation, arbitration, judicial settlement, resort to regional agencies or arrangements, or other peaceful means of their own choice."
Judicial means of dispute resolution are available, most notably in the form of arbitration courts/panels and the ICJ; however, this always requires that the two countries agree to bring their dispute before such a body. It is important to keep in mind that there is no "World Court". If country A believes that country B violates a treaty and B refuses to take action, then that's it. This might be cause for political sanctions by other countries, but there is no body that could somehow "make" B comply with its treaty obligations.