The resting potential is a nerve cell's (or neuron's) state of electropotential charge when it is in a state of rest ready to receive a nerve impulse (the action potential). The resting potential is a state of disequillibrium where the surrounding extracellular fluid has a positive charge and the neuron itself has a negative charge. The charge difference is created by positively charged particles (potassium and sodium ions) outside the cell which are kept apart from negatively charged particles (chlorine and protein ions) by a semi-permeable membrane. Special selective gates in the membrane called Sodium and Potassium Channels let through sodium and potassium ions respectively when they are open. The channels open and close in response to electrical or chemical stimuli.
How the resting potential potential is maintained and how it works in respect to nerve impulses is discussed more fully in the section on the action potential.
Resting Potential links:
Psyc. 358 Lecture - Resting Potential I
Psyc. 358 Lecture - Resting Potential II
Neuroscience for Kids - Action Potential
The Neuron & The Nervous System An Overview
Transmission of Nervous Impulses