Zotero can import bibliographic data stored in a variety of standardized formats used by databases and other reference management tools. The most popular formats are RIS, Bib(La)Tex, and MODS.
If you have a database stored in one of these formats, such as a BibTeX database you've compiled or a RIS database you've exported from another reference manager, you can import them into Zotero by clicking File → “Import…” and choosing “A file”.
See also Moving to Zotero.