When a spring is compressed or stretched, the force it exerts is proportional to its change in length. The rate or spring constant of a spring is the change in the force it exerts, divided by the change in deflection of the spring. That is, it is the gradient of the force versus deflection curve. An extension or compression spring has units of force divided by distance, for example lbf/in or N/m. Torsion springs have units of force multiplied by distance divided by angle, such as N·m. The inverse of spring rate is compliance, that is: if a spring has a rate of 10 N/mm, it has a compliance of 0.1 mm/N. The stiffness (or rate) of springs in parallel is additive, as is the compliance of springs in series.

Depending on the design and required operating environment, any material can be used to construct a spring, so long as the material has the required combination of rigidity and elasticity.

Springs can be classified depending on how the load force is applied to them:

Tension/Extension spring - the spring is designed to operate with a tension load, so the spring stretches as the load is applied to it.

Compression spring - is designed to operate with a compression load, so the spring gets shorter as the load is applied to it.

Torsion spring - unlike the above types in which the load is an axial force, the load applied to a torsion spring is a torque or twisting force, and the end of the spring rotates through an angle as the load is applied.

They can also be classified based on their shape:

• Coil spring - this type is made of a coil or helix of wire
• Flat spring - this type is made of a flat or conical shaped piece of metal.

The most common types of spring are:

• Cantilever spring - a spring which is fixed only at one end.
• Coil spring or helical spring - a spring (made by winding a wire around a cylinder) and the conical spring - these are types of torsion spring, because the wire itself is twisted when the spring is compressed or stretched. These are in turn of two types:
• Compression springs are designed to become shorter when loaded. Their turns (loops) are not touching in the unloaded position, and they need no attachment points.
• A volute spring is a compression spring in the form of a cone, designed so that under compression the coils are not forced against each other, thus permitting longer travel.
• Tension or extension springs are designed to become longer under load. Their turns (loops) are normally touching in the unloaded position, and they have a hook, eye or some other means of attachment at each end.

Other types include:

• Belleville washer or Belleville spring - a disc shaped spring commonly used to apply tension to a bolt (and also in the initiation mechanism of pressure-activated landmines).
• Constant-force spring — a tightly rolled ribbon that exerts a nearly constant force as it is unrolled.
• Gas spring - a volume of gas which is compressed.
• Ideal Spring - the notional spring used in physics: it has no weight, mass, or damping losses.
• Mainspring - a spiral ribbon shaped spring used as a power source in watches, clocks, music boxes, windup toys, and mechanically powered flashlights
• Rubber Band - a tension spring where energy is stored by stretching the material.
• Spring washer - used to apply a constant tensile force along the axis of a fastener.