Tags are a powerful yet often underutilized features of OS X Yosemite. With them, you can easily group even disparate files to keep them organized and make them easier to find later. There's even a great way to make your tags easily accessible — put them in the Dock.
Tags are metadata you apply to your existing files to make it easier to find them later. You can create whatever tags you need, and use them where it makes sense. Working on a work file? Tag it with the "work" tag. Working on a specific project? Add the name of the project. Have to keep track of letters you write to the school about your kids? Embed their names in the tags — it'll all make it easier to find later.
You can add tags when you save a file by adding them right underneath the name of the file. But if you forget, or if you don't think of it just then, you can add tags in the Finder too, by selecting the files or files you'd like to tag and control-clicking on them. The contextual menu that appears enables you to add tags to single files or multiple files you've selected.
Assuming you already have a lot of tags already embedded in your files, you can find them easily enough by clicking the tag name in any Finder window sidebar. Files matching that tag will appear.
That's fine, but it's still an extra step if you need to find files: You have to go to the Finder, open up a new window, then search for the tag you're looking for. You can find the tags in the Finder sidebar beneath your list of favorites.
To more quickly find tagged files, you can simply drag that tag from the Finder sidebar to the right side of the Dock divider, wherever you'd like to place it. The Dock will then create a stack containing all of the files that contain that tag.
To get a quick window showing you all the tagged files, just click on the tag in the Dock then click the topmost icon — the one that has a file count with the words "More in Finder."
That's all there is to it. You can drag as many tags as you want into the Dock. Let me know if you have any questions.