Basically two types: First, full-text databases have the articles attached to the citation, but may index a smaller number of journals.
Second, citation databases, which index a large number of journals, but you may have to use interlibrary loan to find the articles.
In the last 10 years, scholarly publishing has been plagued by explosive growth of bogus scientific journals, sometimes called "predatory journals."
Library databases will screen out these fake journals for you, but if you are picking something from the public internet, you need to add an extra step to your research. You need to make sure the article you are looking at comes from a genuine peer-reviewed journal.
There is a library database called Cabell's that lists predatory journals: Cabell's Predatory Reports. Go to this database and type in the name of the journal you found on the web. If you find the journal in this database, you should not use the article in your research.