I am working on a project with Arduino and I need an Ethernet shield. but after searching in some electronics shops I found there isn't only one but a lot. I don't know which one is better and what are the differences.
Arduino Ethernet with PoE module
Ethernet Shield for Arduino - W5200
Arduino Ethernet Rev3 Without PoE Module
Ethernet Shield W5100 For Arduino
in DFRobot Online Shop.
Clearly the 'best' one is the one that meets your requirements (or is the most expensive - that's always the best, right?).
You say you need an ethernet shield - can you explain why? Without that information, it's almost impossible to tell you which is the most suitable.
If you are looking at IoT applications, it may be that an Ethernet shield isn't the 'best' thing for you. If you just want access to the local IP network, you could use any of those. If you want the 'best' library of functions to support your application, that may affect which module you use.
For me, I think the best module is the cheapest one that gives me connectivity: http://www.aliexpress.com/item/Hot-Sale-ENC28J60-Ethernet-LAN-Network-Module-For-Arduino-51-AVR-STM32-LPC/32254842414.html
The W5100 is also a good choice, but only if it is under $7
What one that is the best is highly subjective.
Though my personal experience is that arduinos original is the one with the best support. Where you can find alot of documentation about the other ones, you might need to do some legwork updating the code to the latest version of the arduino ide or use an older ide.
Be aware that some Arduinos has integrated ethernet, and others are SHIELDS, you recognize the arduinos with ethernet by the power-connector, and the shields by the distinct name containing "Shield", though shields require an arduino to work. The arduino with integrated ethernet does not.
Also be aware that not all shields fits arduinos boxes if you want to encase it. And of cause, buying arduinos original products, cost a bit more - but you support the arduino foundation, and further development of the products.
