Open access journals are journals whose content (articles) are freely available on the Internet. Both large and small publishers publish open access journals. The basic difference between open access journals and subscription-based journals is the financial model they use to finance their publishing. The free vs. subscription-based access to their articles. Some open access journals have author fees (APC - Article Processing Charges) while others are financed by a university or research organization and there are no fees involved. There are also membership models, for example, Biomed Central's membership model.
The umbrella term for these open access journals is usually Gold Open Access but they are two different types based on their fee vs. no fee aspect:
DOAJ is a database for open access journals. The journals that are listed in DOAJ have undergone a qualitative vetting process based on aspects such as the journals' peer review process, editorial board, contact information etc. See more about this here. Besides this, you can also search for open access articles. DOAJ can answer questions like these:
When you search in DOAJ you can see in the search list whether the journal has any author fee or not:
If you click on the journal-title you will get a link to the journal website and more basic information, such as publisher, country, keywords describing its content, author fees and other editorial information:
Principles of Transparency and Best Practice in Scholarly Publishing
A description of DOAJ's quality assessment process of the journals indexed in the database
Ulrichsweb is a database of journals and other periodical literature where you also can limit your search to only open access. Remember that Ulrichsweb indexes more or less all journals and doesn't make any qualitative selection (see further in the sections about Questionable publisher and journals) so therefore you need to check the potential journals, for example via the database DOAJ (see above).
In Ulrichsweb you click on the link Advanced Search:
Click on More Limiters:
In the box Key Feature, select Open Access and click on Search:
In the database Web of Science you can in the search results list limit to only open access journals. The limit functions are available in the column left to the search results. Tick the box in front of Open Access - YES:
In the database Scopus there is an option called Sources > Browse Sources where you can search for journals and limit to only open access journals:
In the database Journal Citation Reports, where you can search for the Impact Factors of Web of Science indexed journals (more about this database is found in the section Impact factors: Web of Science), you can also limit your search to open access journals only:
In the database Scimago Journal & Country Rank where you search for impact factors journals indexed in Scopus (more about this database can be found in the section Impact factors: Scopus) you can also limit to only open access journals: