Slip Laws: The first step of a statute becoming a law, printed on a pamphlet and not yet codified. Only start your research here if the law is very, very recent.
Session Laws: The second step of a statute's life cycle, still not yet codified. Here, the slip laws are organized chronologically and "bound" together (federally, in the Statutes at Large). Only start your research here if the law is very recent.
Code: The official codification of the law, and easier to use than slip laws and session laws, but it's only the letter of the law with no additions.
Annotated Code: Just like the previous code, but filled with extra annotations that can help you find more relevant information like history, cases, etc. Start your research here if possible.
You can find a free version of the US Code here (provided by the US Office of Law Revision Counsel), though it isn't annotated and is only the letter of the law. You can find a free, un-annotated version of the Texas Code here (provided by the Texas Legislature). For annotated codes, you'll most likely find these on paid subscription services like Westlaw and Lexis. You should start your research with annotated codes if possible, but always remember to cite to the un-annotated version of the code.
Annotations can be found on both Westlaw and Lexis within each code section, on the toolbar either just above or just beneath the title of the code section. The annotations are most often condensed into drop-down menus entitled "History", "Citing References", or "Citing Decisions".
Historical & Statutory Notes: References to other/older versions of the law, amendments that affect the statute in question, or other documents referencing the history of the statute's construction.
Cross References: References to other parts of the same code, usually explaining nuances like definitions or interpretations of phrases.
Notes of Decisions: Concise summaries that capture key points or rulings made by a court in cases interpreting the statute. These provide valuable insights into how a codified law has been interpreted, applied, and clarified by the courts.
Administrative Code References: References to other administrative codes, usually state agency rules (like the Texas Administrative Code).
Research References: Extra references that help explain the statute, usually secondary sources that can explain the interpretations of the statute and point out further important information or lead you to more statutes and cases.
Table of Contents: On both Westlaw and Lexis, there is a table of contents button on the top left of the document screen. Sometimes you'll search for a particular statutory section and find that the answer you're looking for is in a different part of the statute. The table of contents can help you browse through the statute's sections easily, so you can find context or definitions without struggle. You can use the table of contents to browse the entire code, to browse each title of a code, or even specific parts of a chapter.
Index: Westlaw has created dozens of indices for federal and state statutory codes. For example, when you click into "Statutes", and then "United States Code Annotated (USCA)", and then the light blue shaded toolbar labeled "Tools & Resources" at right, you'll see "Index - USCA". This will take you to the USCA Index, where you can browse through different code sections using keywords and basic concepts.
Popular Names Table: Westlaw also has compiled lists of the popular names of acts, including both the short title of the act and its popular colloquial names (e.g., Affordable Care Act = Obamacare) at both the state and federal level. You can find the Popular Names Table in the same "Tools & Resources" section indicated in the paragraph above.
Advanced Searching: Both Westlaw and Lexis support Boolean (terms & connectors) searching, and both have advanced searching to help you create a narrowed search. These advanced search forms can be accessed by using the "Advanced Search" function to the right of the main search bars.