When upgrading to Yosemite, a new icon will appear in your dock. It is for Apple's replacement for iPhoto, and it is called Photos. If you had iPhoto in the dock, it is now gone.
Launching Photos can do one of two things. If you have just one iPhoto library, and it is in the default location, it will start migrating it to Photos. If you have more than one, you will be prompted to select the one you want to migrate. This process can take a very long time if you have a large iPhoto library, or even a moderately sized one on an older Mac.
When the process finishes, you will want to go to the VIEW menu in Photos, and select Show Sidebar, so that Photos will start to resemble iPhoto. It can still be confusing to figure out this new organization. Your Albums should be available, and Events have now been converted to Albums as well, and are located in a folder called Event Albums.
But what happens to your old iPhoto library? If you look in your User/Pictures folder (the default location), you will see it is still there, but there is also a Photos library there as well. It appears that the iPhoto library has been duplicated. If the iPhoto library was 100GB, the new Photos library also appears to be about 100GB. Are there really two libraries now using up 200GB? Actually, no. Both libraries combined will take up about 100GB of space.
Apple has used a very clever method to link the two libraries (called hard links). Despite appearances, there is only one copy of each photo and video. It is literally in both places at the same time.
On some Macs, the old iPhoto application has a slash through it, signifying it won't work any longer. On other Macs, iPhoto is still fully functional. But what happens to these two "identical" libraries if you start using iPhoto? That depends. On some machines, iPhoto just won't open, giving you an error message. On others, it will open and you can add or delete videos and images from it.
Now herein lies the rub. If you continue to use iPhoto, its library will have different content than the Photos library. New content won't be reflected in the Photos library. Nor will any deleted content be removed from the Photos library. In essence, the libraries will become "forked."
So the first thing to know is that you do not want to use iPhoto ever again once you have migrated your library to Photos. If you have done that, then you will want the content that is only located in the iPhoto library moved (imported) to the Photos library. After that, you will want to delete the iPhoto library.
This can be done in a few different ways. You may want to schedule an onsite appointment with me to help you get this all settled down.