[iMore.com]

If you take lots of photos, or you want to keep specific photos separate, you can set up and use multiple libraries.
Whether you want to break up an incredibly large library into more manageable parts, or you want to keep personal and professional pictures and videos separate, or iPhone vs. DSLR, or you want each family member to have their own library, you can easily set up and use them in Photos for OS X.

Full story from the iMore Blog...