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 an open-source product, Arduino has become popular in almost all workshops, so demand is enormous. Since building plans are public, other electronics manufacturers started building Arduino compatible boards. These are the same Arduino boards, but with Arduino’s name and logo removed. It 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 with Arduino functionality 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 a USB to serial converter. Using AVR chip gives much more flexibility to use Arduino as different USB peripheral like HID-compliant mouse, keyboard, or memory. Since UNO’s appearance, it has already passed three revisions. 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. A more significant difference was replacing Atmega8U2 with Atmega16U2, which gives more space for building custom USB applications.
Of course, the UNO board still carries Atmega328, the main microcontroller, 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 a 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.