Arduino programs can be divided in three main parts: structure, values (variables and constants), and functions. The Arduino language is based on C/C++.
Variables are expressions that you can use in programs to store values, such as a sensor reading from an analog pin.
Constants are particular values with specific meanings.
Variables can have various types, which are described below.
Used for communication between the Arduino board and a computer or other devices. This communication happens via the Arduino board's serial or USB connection and on digital pins 0 (RX) and 1 (TX). Thus, if you use these functions, you cannot also use pins 0 and 1 for digital i/o.