If you have multiple PCs in your household, there may be some files that you'd like to be able to view/update on both machines. In order to keep your files in sync, there are many tools available.
I personally use a couple of different tools. I use SyncToy from Microsoft to keep my files synced between a flash drive and my hard drive. It's free and easy to use, very similar to Windows Explorer. You choose the "from" location and the "to" location and it compares the files and updates what is needed. It has several different options if you want to copy files only one way, both ways, including deletes or not.
I use DropBox to share files with clients or with family members. I've also found that DropBox is an easy way to get my photos off of my iPhone or iPad and copy them onto my PC without having to email each file to myself. It is also easy to use and offers some space for free. If you invite others to join a shared folder with you, or perform certain other functions, you can earn additional free space.
Here is an article from PC World on this topic which has some other suggestions.
For more information on SyncToy and DropBox, see these links: