By creating custom labels for your project items, you can filter in project views to see a group of items that have been labeled a particular way, and you can also search for items that share the same label across projects.

The simplest way to label items, when in a project, is from the table view. Here's where to click and type:

Once you've tagged items, click filter under the project title, then click the tags filter box to find all items in the project with a specific label.

You can also search for tagged items across projects by going to the team alignment area, then reports.