As Paul mentioned, they are usually completed in sequence, with the Master's following completion of a bachelor's, but some schools also offer dual programs where you complete them together in a 5-year program. Depending on the field of study, advanced degrees usually bring two benefits: increased knowledge about a given field and increased job opportunities.
During an advanced degree program, you will be taking additional classes, but often times there is a research component associated with it. In science and engineering fields, that might be conducting experiments or proving out a new design. For a humanities study, that might be compiling existing papers and documentations, and coming up with new conclusions and observations. While some bachelor's programs include a thesis requirement (usually a lengthy paper or capstone project), nearly all master's programs have that as a requirement to graduate. All of this extra work helps to become more of an expert in a field.
As far as earning potential, in general, graduates with advanced degrees can obtain better jobs, and are paid more over the course of their lifetimes. However, that's not always the case and some master's programs are very expensive, meaning that on top of paying thousands of dollars to attend the school and missing out on 2+ years of working, you may not be able to make more than if you had not attended graduate school. The key is to research what typical students from the programs you're looking to attend earn after they graduate and if that's the path that you want to follow. In my case, I did graduate from a master's program in engineering, and I've seen the value of the degree to be worth it in my career. However, that is not the case for everyone.
Best of luck!