ADC for low signal (microvolts)
The most basic parameter of an ADC is it's resolution and sampling speed. 

Sampling speed is pretty obvious - it tells you how often a sample (digital value) is produced. It's expressed in Hz. This determines if you can detect fast changes in the signal.

The other thing is resolution. It's expressed in bits. If you have a 8-bit converter that means that it produces digital values between 0-255 (or 2^8=256 values). If your ADC has a 5V reference voltage (or to simplify in the case of an Arduino, a 5V power supply) that means that the entire range is divided into 256 equal parts. So 5V/(2^8)=0.02V is the smallest value your ADC can detect (i.e. 0x00 => 0V, 0x01 => 0.02V, 0x02 => 0.04V and so on).

The arduino has a 10-bit ADC so that gives 5V/(2^10)=0.00488V. If you want more resolution you can look for how much bits you need. In your case: 5/(2^14)= 0.000305V would be enough. However I think that it might be easier to find and audio range ADC chip with an SPI bus output to communicate with the arduino (usually audio ADCs use the I2S bus for output). Those would go to 16-bits, so you'll be well covered. 

Alternatively, if your signal is slow and you don't need fast sampling speeds, you can go for data acquisition chips like ADS1100 of ADS1113. The sampling rate is in the order of a few hundred Hz.