Wikipedia is the encyclopedia itself, while Wikimedia Commons is the Wikimedia community's free media library. Commons hosts freely licensed images, videos, and other files that can be used across all Wikimedia projects. All freely licensed or public domain files—such as headshots, event photos, or cityscapes released under Creative Commons—must be uploaded to Commons.
By contrast, non-free content such as company logos or book covers must be uploaded directly to Wikipedia with a strong fair use argument. These files are hosted locally, can only be used in specific articles, and require detailed justification.
In short: Commons supports open-access media across all Wikimedia projects, while Wikipedia itself only permits non-free media when necessary to represent the topic clearly.
