Espressif Systems ESP8266 Beginner's Manual page 5

Before you begin ...
There's a few things you have to look out for when using an ESP8266: The most important thing is that it runs at 3.3V, so if you
connect it to a 5V power supply, you'll kill it. Unlike some 3.3V Arduino or Teensy boards, the ESP8266's I/O pins are not 5V
tolerant, so if you use a 5V USB-to-Serial converter, or 5V sensors etc. you'll blow it up.
A second thing to keep in mind is that the ESP8266 can only source or sink 12mA per output pin, compared to 20-40mA for most
Arduinos.
The ESP8266 has one analog to digital converter, but it has a strange voltage range: 0 - 1V, voltages above 1V might damage the
board.
One last thing to keep in mind is that the ESP8266 has to share the system resources and CPU time between your sketch and the Wi-Fi
driver. Also, features like PWM, interrupts or I²C are emulated in software, most Arduinos on the other hand, have dedicated hardware
parts for these tasks.
For most applications however, this is not too much of an issue.