The Toolbar across the top of Yosemite Finder windows.
First of all, here's what you see now: From left to right, we have two arrows for forward and back through navigation, then a block of four buttons for switching views. Then we see a small row of squares on a button, a gear, a share button, and a tag button. Closing out the far right hand side of the Toolbar is the search box.
Arrows are a convenient way to navigate, but you can also use the Command key with either the left or right bracket to move through the folders and files on your Mac. If you're interested in changing the layout of the window you're looking at, the next four buttons switch Finder views so you can find the best one for each case, depending what you want to do.
Your next button is the Arrange button (with the tiny squares), since it gives you a list of items you can sort by, and shows a checkmark on the one you're currently using. If you have a view without header columns this is a quick way to verify how you sorted your window.
Then we have the gear, which Apple has named the Action button. It is the same as the right-click menu, showing options for opening and moving to trash and the like. With the Share button you can add a file to a message in Mail or send it via Messages, or share it to Facebook or other services.
Finally we see the tag menu, where you can assign a tag or a color label to the item selected.
What if none of these are things you use? What if you really need a New Folder button or a Delete button or a shortcut for connecting to something remotely? This is the beauty of that Toolbar: You can customize it! Go to the View menu and choose Customize Toolbar:
Click on View in the Finder, and select Customize Toolbar.
Then you can add or remove items such as Dropbox, Connect (to remote locations), Get Info, Quick Look, and more. Adjust them in the order you want, and you can finally have something useful up there to help with Finder file navigation and management:
All the options available to customize the Toolbar.
There are others that can be added if you have utilities installed which include them, like my Dropbox option in the image above. Now you can adjust and remove the ones you don't like, and make the ones that remain actually be helpful in getting things done on your Mac. Like so:
Here's a customized Toolbar that is actually useful!
Now I have some useful things in my Toolbar. I took most things out and added Path, Get Info, and Quick Look on the left. Over on the right I have Connect and Share for when I need access to other things.