Students are usually eligible to work while in university. This not only provides them with an income, it provides practical life experience and starts building their work experience which could help when they start applying for internships and research opportunities.
If you are on a student visa, make sure you know the regulations for working while in a country as a student.
Visa requirements usually limit students to 20 hours a week while in school. This is not only a visa limit but strongly encouraged for citizens too. You are a full time student so you need to be able to do school 40+ hours a week.
Many countries allow students to work off campus as long as they stay within the limits of the hours.
The US does not allow students to earn an income off campus. Most universities understand this limitation and work to ensure internationals have opportunities to work on campus during the school year and during breaks.
On a student's financial aid package, one of the lines might be "Work-Study", "Federal Work Study" "Campus Employment" or something similar. If you qualify for US federal aid (US Citizens and residents), there is a government program that helps pay for students to have jobs on campus that you might qualify for. Even if you do not qualify for US federal aid, most universities have campus job options and might include it on you financial aid package.
A few things to know about this:
You have to look for the jobs
You have to apply and qualify for the jobs, they aren't just given to you.
The amount on the financial aid package is not exact. You get paid an hourly wage for work performed.
Most jobs are minumum wage. Minimum wage varies by state.
The money can go directly into your bank or into your school account to pay off fees and tuition.
Many recommend that you budget this money for books, pocket money, and maybe money to travel home.