Probably you have noticed that Arduino is spread worldwide. It would be ridiculous to see one manufacturer producing them. The Arduino group produces an original board that is named Arduino. This is their trademark. But being open source product, Arduino become popular in almost all workshops and so demand is enormous. Since building plans are public, other electronics manufacturers started building Arduino compatible boards. These are same Arduino boards, but with Arduino name and logo removed. Other than that is 100% compatible with standard Arduino that works with software tools, extensions, and modules.
Arduino compatible Uno R3 Rev3 development board is one of the most known boards. Because this form factor is the first Arduino step into the community, it became a reference shape for majority shields and other products. Even today great boards that have an Arduino functionality in them still carry this pin arrangement to accept shields. UNO evolved from Duemilanove boards that carry FTDI based USB to serial driver chips. UNO has Atmega16U2 instead which is programmed as USB to serial converter. Using AVR chip here gives much more flexibility to use Arduino as different USB peripheral like HID-compliant mouse, keyboard or memory. Since UNO appearance it already passed three revision. This is why today we see UNO Rev3 boards. The last one had several essential improvements. First of all, they added additional SDA and SCL pins next to AREF. Additionally, they added the IOREF pin which allows adapting voltage to shields from the board. Also, there is another not used pin that might be used in future revisions. There has been a small change in the reset circuit to make it stronger. And probably more significant difference was replacing Atmega8U2 with Atmega16U2 which gives more space for building custom USB applications. Of course UNO board still caries Atmega328 the main microcontroller which is enough for necessary projects. But eventually, you may run out of internal flash or SRAM memory or just lack of performance. You can always switch to more powerful Arduino boards that are compatible with existing shields. You can select bigger Atmega2560 Arduino or go with Arm-based Due. Looking for cheaper Arduino compatible boards – check out Gearbest.com they offer great discounts on their products.