What is the meaning of internal Energy? What is the meaning of internal energy. Is it the amount of energy that a body contains?
 A: Standard Definition : The internal energy of a system is identified with the random, disordered motion of molecules; the total (internal) energy in a system includes potential and kinetic energy.
Since this molecular motion is primarily a function of temperature, the internal energy is sometimes called 'thermal energy'. 
Total energy of the system $(E)$ = U (Thermal energy) + $E_{Kinetic\space of\space system}$ + $E_{Potential\space of\space system}$  ....
Here, the two terms viz, $E_{Kinetic\space of\space system}$ and $E_{Potential\space of\space system}$, are for the system and not the particles inside the system. For the particles the $KE$ and $PE$ is included in the internal energy.
In most of the cases, kinetic energy, potential energy, chemical energy of the system is constant, thus we conclude,
$\Delta{E}=\Delta{U}$
The proper form of the first law of thermodynamics is given by,
$\Delta E = q + W$,
but for the reasons stated above,
$\Delta U = q + W$.
Therefore, summing it up, Internal energy includes :
1). Translational KE
2). Vibrational and rotational kinetic energy
3). Potential energy from intermolecular forces 
