As an OS X user you likely have at least some of your photos stored in Apple’s provided Photos application, and by doing so, you can use them with iCloud services, and in applications that interface with OS X’s media sharing services. However, there might be times where you want to keep some photos separate from others, and manage them in more private ways than having them accessible by other applications and services.
Creating a new library
Creating a new library for your various projects is relatively simple. First quit Photos, then launch it while holding the Option key, and you should see the library manager window appear. In here, you should see your current library listed, but also have the option to create a new library. Once created, you can then drag and drop or otherwise import the photos you want, and this new library will be the default one used when Photos launches.
With multiple libraries on your Mac, you can switch between them very easily using two methods. The first is using the same library manager, so simply quit and relaunch Photos with the Option key held, and you can then select your library. However, an even easier approach is to simply open any of your photo libraries directly in the Finder, and it will open in Photos and become the default library used. With this approach, simply place the libraries or aliases to them in a convenient location, and then open them directly, instead of opening the Photos application itself.
The real issue with multiple libraries is not which library is the default opened with Photos, but which is used as your System Library, that is, the one that third-party services like iCloud and other applications will reference when they are instructed to access your photo library. To change this, you need to go to the Photos preferences and click the button to use the current library as the System Library.
The caveat here is that in order to use a library with iCloud and other services, it must be stored on a volume that is formatted to Apple’s native HFS+ (Mac OS Extended) format. You can check this on any locally-attached drive by selecting it in the Finder and pressing Command-i to reveal the information window. Then expand the General section, where you should see its format listed.