Difference between "2-year community or junior colleges" and "4 year undergraduate program" in USA
Two-year degrees in the US often award either an "associates degree" or some technical certification/profession-specific degree.
These degrees are not typically part of the "bachelors -> masters -> PhD" pipeline or more generic "bachelors -> professional degree".
Some students may start taking courses at a 2-year school and then transfer to a 4-year school (with or without actually completing a 2-year degree, and hoping to have as many of their 2-year credits count/transfer towards their 4-year degree), because 2-year schools are both less expensive and less selective. 2-year programs rarely have any sort of on-site housing ("dorms") and limited "student life" opportunities (clubs, social facilities, athletics).
Not all 2-year schools are appropriate for this track. Some are specifically designed as technical degrees to get students into a specific career, and do not provide the "general education" credits (basic math, science, social studies courses) that are useful for transferring to a 4-year program.
If you wanted a MSc in the US, starting from high school you would either:
A) Enroll at a 4-year program, and then
B) Apply to Masters programs*
or
A) Enroll in a 2-year program,
B) Transfer to a 4-year program (bringing with you some or all of the credits you already earned), and then
C) Apply to Masters programs*
If you plan for the "ABC" pipeline it is definitely worth exploring the requirements of the 4-year institution(s) you plan to attend so that you know which courses you take will transfer (some 4-year schools have explicit relationships with particular 2-year schools to define classes that will count for transfer credit). Most students who would follow this path instead of the "AB" path would be doing so either for financial reasons or because the student does not feel academically prepared for a 4-year school (the quality of high school education varies quite a bit in the US, and even a very good student at a disadvantaged high school might have trouble with the jump to a 4-year school).
There is no particular advantage to completing a 2-year degree program before starting a fresh 4-year degree - the only reason to start at a 2-year program if your goal is a MSc would be if you can transfer credits from the 2-year to 4-year program.
(* - it's possible, though not particularly common in the US, that you enroll directly for a combined bachelors+masters program)
A community college or junior college in the US offers a program that is roughly equivalent to the first two year of a bachelor's program. The degree awarded on successful completion is normally called an Associates Degree. In most cases it omits the more specialized "upper division" courses that bachelor's degree students take in the last two years.
An associates degree would normally not be sufficient to gain access to a masters program. The student would need to complete a bachelors first.
The advantage, for many, of an associates degree is that the college might be closer to home and it might have a much lower tuition cost than a bachelors degree program. As implied in the name "community college", many of them are partially funded by community taxes. Some of these colleges are very good, actually. I know people at a few of them and have high respect for what they do. But they aren't involved in any kind of serious research for the most part.
Some community colleges also offer non-academic training in various trades such as plumbing and carpentry.
I should also note, that since times are difficult in academia at the moment, some quite highly skilled "academics" wind up teaching in community colleges. One can build a career there if you are willing to focus almost everything on teaching.
The other answers are largely correct. For those with higher-academia goals, community colleges provide Associates Degrees, and opportunities to apply those credits directly to a later 4-year degree. The primary reasons students attend these institutions are (a) lower annual cost than 4-year schools, and/or (b) students who cannot satisfy 4-year college admissions requirements, and hope to establish a track record that allows them to do so later.
That said, there are variations in institutions that the other answers don't cover. For example, I work at a community college in the CUNY (City University of New York) system, one of 25 campuses, including both 2-year and 4-year colleges, around the city. The three things I'd highlight at our schools are:
While the 4-year schools have significant admissions requirements, the 2-year schools are open admissions; any student with a high-school diploma or the equivalent is guaranteed placement.
A student who earns an appropriate Associate's Degree at a 2-year school is then guaranteed placement at a 4-year school within the system, with all earned credits transferring over. So the 2-year degree serves as successful completion of the first 2 years of a 4-year degree. (Even if the student could not initially meet the admissions requirement of the 4-year school.)
Faculty throughout the university system (2-year and 4-year) are all held to the same tenure requirements, including the need for published research. Admittedly, this is quite different from most other community colleges, where research is not a requirement.
In short, the 2-year school has historically provided an opportunity (maybe a "last chance") for a student to on-ramp to an academic path which will later take them to a Bachelor's, Master's, and perhaps Doctorate degree, and at a reduced cost for the first two years. That said: there are some rumors, in the current economic crisis, and based on perennially low success rates, that this on-ramp opportunity to higher education should be refocused to immediate workforce placement after the 2-year degree.