MarkDuino is a small (just 58x33mm) and cheap Arduino clone board which is stripped out from unneeded components with some extra features. It has built-in I2C connection which is handy if you want to add LCD display with I2C 1602 LCD Controller out-of-the-box. This board doesn't have voltage regulator so you must provide 5v power (externally) or power it via USB using USB to TTL Converter UART module.
Note: when wiring USB to TTL Converter UART module, wire RXD to TX on board and TXD to RX.
Arduino UNO vs MarkDuino 1.0 (Size Comparison)