I have seen the words college and university sometimes mixed in the same article. But there are lots of sources where only one is used. I've looked up them in various dictionaries, but couldn't figure out the difference in their meaning. So, what is the difference (of meaning) that justifies the above findings regarding their usage?
Both the definitions of "college" and "university" and the meanings of the idioms in which those words appear (e.g. "to go to college", "to attend university"), differ by country (including native-English-speaking countries), so it would be very difficult to give you a good answer here.
I'm afraid the best I can do is refer you to the Wikipedia article College.
It's complex. College as a noun without an article refers to one's undergraduate studies in US English, where most other English speakers (and most other European languages) use uni or university. In that sense university is, for an international audience, a better choice.
It can also, in the US, refer specifically to a tertiary educational institution (as in, "liberal arts college" or "community college") that has the right to issue one or more of bachelor's, master's, or associate's degrees, but not doctorates - a school able to issue doctorates is a university. This concept of college is rather American and difficult to translate even to people in other English speaking countries.
It can also refer to a division of a university, either along lines of disciplines (such as, "College of Arts and Sciences"), or simply smaller groups of students and/or faculty (such as the constituent colleges of Oxford or Cambridge).
Finally, it can refer to a formal group of peers with special rights or characteristics, such as the College of Cardinals of the Catholic Church, the Royal College of Surgeons, or the College of Engineers and Surveyors of Puerto Rico. This use has nothing to do with higher education, although educational attainment may be a necessary or sole requirement for admittance.
A college confers a bachelor degree (usually a four-year program) - a BA (Bachelor of Arts) or BS (Bachelor of Science). There are institutions that offer only bachelor degrees. Universities offer, in addition, a master or a doctoral degree. Colleges are often integrated into universities. So, you can get a BA or BS at a university, but you cannot get an MA or PhD at a college.
The historic answer is an institution may be called a university when it contains at least 4 colleges. It does not have to confer doctoral degrees to be a university; moverover, there are separate individual graduate and professional schools of various disciplines that confer Master's and Doctor's degrees.