If you upgrade to Yosemite 10.10.3, you also automatically get the new Photos app and your iPhoto library is automatically migrated to Photos. Although there are some important bug fixes in the 10.10.3 upgrade, you may for various reasons want to opt out of Photos and continue using iPhoto at least for the short term.

Photos is a good app, but as is Apple’s habit, some very useful capabilities of iPhoto were deleted from the new Photos, notably some batch processing capabilities and the ability to edit, add or copy location data to photos that don’t have it. This is a deal-breaker for me because I have a lot of photos from cameras that don’t have GPS capability, and I intend to stick with iPhoto until Photos recovers this capability.

Apple states that you can continue to use iPhoto, but the details are a little vague. For one thing, you have to have iPhoto 9.6.1.

If you have an older version of iPhoto you need to upgrade. Unfortunately, 9.6.1 is no longer available for download in the App Store. Here is a round-about method that may work.
There is also a warning that any photos or albums you add or edits you make with iPhoto will not be carried over into Photo. The warning is not as dire as it sounds. Sometime in the future when you decide to switch to using Photos, it is quite easy to update Photos with the changes you have made while continuing to use iPhoto.

To continue using iPhoto, find your iPhoto Library. By default it will be in your Home Folder/Pictures. After the automatic migration to Photos, the filename and extension will be iPhoto Library.migratedphotolibrary. (if you have renamed your iPhoto Library, the first part will be different.)

Close Photos if it is open. Double-click the above file. You will get the following dialog box. 

Click on Open iPhoto. From now on the library will open when you launch iPhoto. (The name also gets changed back to iPhoto Library.iphotolibrary.) You can continue to use iPhoto for your photo editing and storage, and ignore Photos until such time as you wish to or need to make the switch. (Photos will also continue to work, but the content will be only the things present when the initial migration from iPhoto to Photos took place.)

When you are ready to abandon iPhoto and switch to Photos, just drag the iPhoto Library.iphotolibrary file to the Photos icon in the Dock or Applications Folder. Your iPhoto Library will be re-imported with all the changes you have made.