Arduino is micro controller board made by Italian company Smart Projects in 2005. its open source device with which we can control various devices, tasks and programs.
Because of its open source its world known and has a big database with lot of users sharing their projects and making device better with spreading amount of its abilities. With all that everyone can put their projects to be available for the whole world to use.
Programming of device can be made with almost any coding program but program "Arduino" is mostly used because its simplicity and it is completely free to use, its based on C/C++ language.
As its open source everyone could use their electric circuits and designs so there are many of types of Arduino devices which vary on number of inputs/outputs, controller chip, other hardware components and dimensions.
Probably most known is Arduino uno rev3 (R3) based on Atmega328P
It has 14 digital input/output pins, 6 analog pins, input voltage 7-12V.
Its connected to computer via usb type B (2.0)
Second is Uno bigger brother Mega 2560.
It has 54 digital input/output pins, 16 analog input pins.
Its mostly used for 3D printers and some bigger projects.
And third most of used ones is Nano
As it name suggest it is the smallest of the three with 22 digital and 8 analog pins with dimensions 18x45mm
There are a lot of other Arduino boards with different applications such ones with wifi module build in, or one that connects with Ethernet port.
And the best part is that if Arduino is not maybe what are you looking for you can add various modules to it, they are called shields we will talk about them in another post.
Source: https://www.arduino.cc/