The in-game UFOPaedia for X-COM contains an expanding amount of information as you progress through the game and complete more research. It is by no means infalliable, but it does contain a large amount of useful information. The UFOpedia is broken down into nine sections which each cover a general area of information. In each case you click on the type of information you are interested in to see a further list of items each of which has a data page to be viewed which will contain different information and statistics depending on the item type investigated.
XCom Craft and Armament
This covers all the Craft you can use in the game, including some technical details of the two that you start with, as well as adding new vehicles that your scientists can develop as your game progresses. Also included in this section are all the various armaments that most of these craft can mount, three at the start and again more as you research them.
Heavy Weapons Platforms
This covers the small autonomous tanks that you can deploy either in base defense or on Alien Missions. There are initially two items in the list, and a large amount of technical information is available regarding them. Further HWPs will become available as the game progresses.
Weapons and Equipment
This section includes a large list even from the start, covering all the basic equipment and Weapons you can purchase at any base. A few extra items of non-alien sourced technology can be added to these lists, as well as all the types of armour that you research will be shown.
This section starts off completely empty, as may be expected, but there are quite a large number of alien items that you can research that will appear here, where you will be able to find some useful statistics about their power and accuracy to compare with the existing weapons you have.
Contains a list of all the Base Facilities you can build, their cost, construction time and maintenance cost, however all this information is available to you each time you build one on the main Base screen. There is also a textual description of the role of the facility. A number of extra types of facility will become available to you with appropriate research.
Alien Life Forms
This section contains information on all the Alien Life Forms you meet, each will end up with both an autopsy report, and a more general report of the alien race, between these two pages a reasonable estimate of each races strengths and weaknesses can be made.
Contains general information about Aliens - parts of their plans you might have discovered, what tasks the aliens sent to earth are instructed to do, and also research into finding a final solution to the alien problem.
Covers all the major parts of alien craft that you shoot down, their propulsion and navigation units, analysis of their construction and materials and a variety of other information. This list starts of empty but even your first UFO downed might already allow you to fill in several items in this list.
Contains information about the various craft that the aliens use to perform their missions on Earth, with such key pieces of information as their top speed and weapons range.