All the arduino "clones" (barring quality control issues, which are a problem with the lowest tier ones) will work for simple stuff.
Main differences between a clone and official:
- Official boards kickback money to Arduino to support the project, clones don't.
- Clone Uno's and Mega's don't usually use the 16u2 for serial interface. The 16u2 can be reprogrammed for advanced functionality like pretending to be a keyboard or mouse.
- Cheapest of the clones often have quality control issues or design errors; they're not recommended for beginners for that reason.
- They're between 2 and 10 times cheaper than the official boards.
* Everything works with the Uno unless it says otherwise.
* Most things work on the Mega
* A lot of things don't work on the Due
Non-shield sensors, modules, etc should work with anything (though the Due requires more work to use with many 5V sensors)
The most common board to start with is the Uno - that's where a lot of us started, and it's the best known and the most has been written for it. It's common to use clones after a project is working on a "nice" board, when you want to button up your project and use the nice board for the next project.