Labels are a handy way to organise your files in the Finder by colour-coding their icons. Until OS X Mavericks is released later this year, they are a pretty good substitute for having a proper way to tag files in the Finder.
Adding a label to a file or folder is simple. Just right-click the icon and choose one of the coloured squares from the menu. Alternatively, you can select the icon by clicking on it, and choose the label from the File menu in the menubar.
If you want to use labels a bit more like tags, it is possible to choose a custom name for each label colour in the Finder Preferences (under Finder in the menubar). Once you have done this, the only limitation compared to a real tagging system is that you are restricted to only having 7 different tags.
Using labels as tags can work really well in conjunction with Saved Searches (also known as Smart Folders).
Most people will have their documents organised into separate folders for each project, and will have separate folders for work and home files. But sometimes, for example, you might want to see all your receipts from all your different projects together, or all the files that you need to print. Instead of duplicating all your receipts into a separate “receipts” folder, you can just use labels along with a Saved Search that displays all the files with that label.
To set up the saved search, start by switching over to the Finder and choosing Find from the File menu (or type Command-F). Choose Other from the left hand drop-down menu, and choose File label from the list. Then just choose the desired label, and click the Save button to add the Saved Search to your sidebar.
Power users can take this idea even further using a tool like Hazel. Firstly, Hazel can automatically add labels to your files based on your own criteria. For example, automatically adding a red label to files that have been in the Downloads folder for more than a week, or adding a purple label to files that were downloaded from your bank website.
Secondly, Hazel can automatically perform actions when you assign a specific label to a file. For example, it could import MP3 files into iTunes if they you label them purple, or resize images to 800x600px and upload them to a server if they are labelled green, or run any AppleScript or shell script you want.
Bonus tip: You can quickly change the label of a file by dragging it into the section for a different label in the folder, as show in the screenshot below. For this to work, you must have your folder in Icon View, and have the folder Arranged by Label.