Most modern GNU/Linux distributions use some kind of a software service that tracks the user activities and events. These events can be anything, from the opening of a document file, to the chat conversation. This isn't happening for the purpose to monitor the user and sell this usage data information to 3rd parties, but to help users enjoy a more user-friendly and unified experience across their applications. For example, if you want to quickly locate that document that you opened last weekend, chances are that you will easily and promptly find it after opening your file manager and going to the “Recent” folder.
Zeitgeist
Zeitgeist is one of the most widely used event loggers in the Linux software world, often even utilized as a central log management system for multiple other applications that send their event logs directly on the zeitgeist daemon without storing them into their own folder/location. The event logger stores this data on /.local/share/zeitgeist. Zeitgeist is known to work with numerous software tools, and I can indicatively mention that it sends and receives metadata to the Unity Dash, Docky, gEdit, Totem, Rhythmbox, Midori, etc.
In order to be able to set the Zeitgeist options, you will need to install the package named “activity-log-manager”. In Ubuntu, open a terminal and enter the following command: “sudo apt-get install activity-log-manager”. Then launch the utility and you'll be provided with a set of options that refer to Zeitgeist's operation.
For example, if you want to disable the daemon you may simply click on the switch that is located on the top left. If you want to exclude a particular category such as chat logs or music playback history you may do so by unchecking the corresponding box on the left side of the application. If you want to exclude a certain folder from being tracked by Zeitgeist, click on the “plus” icon on the bottom and navigate to the folder through the file manager window. You may also exclude an application through this tool by selecting “Add Application” instead of “Add Folder”
If you want to completely erase the history logs kept by Zeitgeist, click on the button entitled “Clear Usage Data...” and select the time period that you want to wipe and then hit “OK” and you're done.
GNOME Tracker
GNOME Tracker is the default “tracker” of the popular desktop environment that indexes files together with their full content. This is especially helpful when wanting to track down a file that you know contains something in particular, but you can't remember its filename. If you are using GNOME Shell, open the tracker search tool and enter the search criteria.
I have entered the keyword “December” and selected the category of “pictures/images” and also selected the “content” choice instead of the filename. This yielded results of photographs that were taken on December and were tagged as such. With tracker, you can open a file and navigate the containing folder as if you were using a normal file manager.
Tracker also features a powerful configuration utility which is launched as a separate individual tool (magnifier with paw icon). On the first tab, you will be given the capacity to set the indexing preferences for Tracker, and even allow it to index files that are contained in removable disks, set resource usage limitations, and adjust the data retainment time threshold.
The next tab is dedicated to the location settings where users can define what folders are monitored. You may add as many as you want by clicking on that “plus” button on the right.
Then you may set what should be ignored by Tracker through the third tab. Here you can add directories, or specific files to ignore.
Finally, you can remove all indexed data and delete everything that Tracker has stored since the beginning of its first run.
If you want to dig deeper into the Tracker's indexed data, you may also do so by installing and running the GNOME Activity Journal. This application makes it easier to find things by navigating the indexed data in a chronological fashion. This application features many different view modes, helping you locate what you're looking for in a more appropriate manner depending on the type of the file.