The official guide/manual for Factorio. Originally written by Xterminator and then taken over and by the Factorio development team. The manual covers the major aspects of the game such as Belt and Inserter mechanics, Science, Enemies/Combat, Logistics System and more. Includes many pictures and links to outside sources, like YouTube videos, informative Reddit posts, forums posts, and more.
Factorio Wiki (not completely up to date)
The Factorio Wiki is managed and update/changed by the community. It is a good source of information for many aspects of the game, but some parts are outdated and incorrect at this time.
The official forums have a very large and helpful community that are always willing to help and answer questions. There are also many helpful topics under the Gameplay Help section, with things from helpful tips and builds, to video tutorial links, and even Combinator & Circuit Network guides and explanations.
A very good train tutorial made in a Google Docs Presentation. It covers everything from the locomotive GUI and what everything does, to Train Stop setup, to Signals, as well as creating junctions and more. Definitely something to check out if you are struggling with Trains.
Additional viewing formats can be found here.