When reviewing a course catalog to determine what courses to sign up for, the first thing you are likely to notice is the course names. The names of courses are often an apparent indicator of the subject matter of the course, and sometimes, which students the course is intended for. For example, a course called "MATH 110: Introduction to Statistics" is straightforward — it's clear from the title that this this an introductory course intended for someone with no prior experience with statistics. Other course titles you will find in the catalog may not be as clear as this, and that is why you need to look beyond the title to decide if a course is right for you.
In many schools, you will find course titles sorted and listed with a three or four-letter abbreviation of the academic department that is offering the course. This is typically followed by a three or four-digit number. For example, "ECON 1001: Introduction to Economics." The numbers are a way of indicating a course's level of difficulty or the level of experience in the subject a student taking the course should have. Course numbers beginning with a "1" are typically introductory or "lower division" courses; courses beginning with a "3" are more advanced and will likely draw more experienced students; courses beginning with a "5" or higher are intended mainly for graduate students. You'll want to understand and consider course numbers when you choose courses and register, not only to decide whether this is a course you want to take, but also whether this is a course you are ready to take.
Course numbers do not merely describe a course's level of difficulty; they also indicate the suggested, or sometimes required, sequence that classes should be taken in. You start with the lower numbers and work your way up to the higher numbers. This is not always the case; for instance, an upper-class student might take an introductory course because they are finishing up degree requirements or simply because they are interested in exploring a subject outside of their major that they have no experience with. Conversely, a lower-class student can take an advanced course, but the difference is that an advanced course may have prerequisites.
If an advanced course has a prerequisite or prerequisites, this means there is a course or courses you have to have taken before you can enroll in the course. For example, in order to register for a class titled "FILM 5010: Advanced Film Seminar," you may need to have taken the prerequisite, "FILM 2000: Introduction to Filmmaking." Prerequisites are designed to ensure that students are properly prepared for the challenge of upper division courses.
You can find course descriptions in a course catalog, on the website of a particular academic department, or other places where course information is provided. A course description includes a number of important details about a given course, including when it is offered, how many credits it is worth, whether there are any prerequisites, and a brief description of the content the course will cover. As it collects all the relevant basic information you will need to make a decision about whether you want to take a course, a course description is one of the most important resources in selecting a course.