What is internal energy?

The internal energy (or thermal energy) of a substance is the amount of kinetic and potential energy (KE and PE) it possesses.

KE and PE normally increases as temperature rises. An increase of PE will increase the separation between molecules against their intermolecular attractive force. The strength of this intermolecular force varies on different substances because of different molecular structures. Thus the gain in PE for the same rise in temperature is also different in different substances. This accounts for the variation of specific heat capacity among various substances.

For example, for one degree Celsius rise in temperature, the gain in PE for water molecules is much higher than the gain in PE for, say copper molecules, for the same mass of materials.
[note: PE, being a form of energy, is expressed in the usual energy unit, i.e. Joule]

Kinetic energy accounts for the motion (both translational and vibrational) of molecules. In gases, the molecules are free to move, and so are moving randomly in all directions. In solids, molecules are bound by intermolecular forces, their motion is limited mostly to vibrational motion. In liquid, although molecules could move around, they are still bound by intermolecular forcea and the range of their motion is quite limited.