If you use EndNote on different computers, you may want to sync your library from one machine to the other. One way is to just overwrite the library on one computer with the latest version that you've saved to a flash drive or emailed to yourself. Another option is to use EndNote Online (formerly Web), an online back-up for your EndNote libraries that allows you to sync libraries online from one computer and then re-sync to another computer with the same EndNote Online account. This is suitable for a home/work computer set up.
WARNING: Do not store your active EndNote libraries in cloud locations, such as Box, Dropbox or OneDrive/SharePoint. EndNote cautions users that libraries stored in cloud locations may become corrupted and unusable. It is best to use EndNote Online to sync your library across multiple computers.
The Northwestern University Library has produced guides showing how to set up the EndNote Web account and sync references. Another excellent guide is available here.