Most simply put, the difference between immigration law and immigration policy is that immigration law determines the legality of immigration and immigration policy is the attitude or attitudes expressed by political leaders. The interplay of the two can be quite interesting as policy may inform when and how the law is applied.
Immigration law is the body of law that pertains to the regulation of the admission, residency, and deportation of foreign nationals. In the United States, federal immigration law preempts state law, which means that the individual states do not create their own immigration law. Thus, immigration law in the U.S. is derived from the U.S. Constitution, treaties, the federal code, and the federal common law. The actual application of federal immigration law is governed by the U.S. Federal Regulations (administrative law), and the enforcement of immigration law is within the scope of federal agencies. All of these, from the laws to the regulations to the enforcement thereof, are aspects of immigration law.
Immigration policy may refer to a variety things, from a nebulous societal construct to the specific policy of the current executive. In the context of the interplay between immigration law and immigration policy, it is probably best to focus on the policy of the executive branch of the government.
As noted above, immigration law is enforced by federal agencies. These agencies are part of the executive branch of the federal government. The immigration policy of the executive branch influences the interpretation and the enforcement of the law by such agencies. It may even affect the administrative law aspect of immigration law, as the agencies, not the legislative branch, are responsible for crafting and modifying the federal regulations that control how federal agencies apply the laws.
So while immigration law and immigration policy are two distinctive things, they work together to determine how the government actually handles immigration issues. The laws provide the legal structure whereas the policy provides the guidance to federal agencies on how they are to interpret and enforce the law.