From BlenderWiki

Follow Path Constraint

Mode: Object and Pose modes

Panel: Constraints (Object context and sub-context, F7, or Editing context, F9, when in Pose mode)

Description

The Follow Path constraint places its owner onto a curve target object, and make it move along this curve (or path). It can also affect its owner’s rotation to follow the curve’s bends, when the Follow Curve option is enabled.

The owner is always evaluated in the global (world) space:

Its location (as shown in the Transform Properties panel, N) is used as offset from its normal position on the path. E.g. if you have an owner with the (1.0, 1.0, 0.0) location, it will be one BU away from its normal position on the curve, along the X and Y axis. Hence, if you want your owner on its target path, clear its location (AltG)!
This location offset is also proportionally affected by the scale of the target curve. Taking the same (1.0, 1.0, 0.0) offset as above, if the curve has a scale of (2.0, 1.0, 1.0), the owner will be offset of two BU along the X axis (and one along the Y one)…
When the Curve Follow option is enabled, its rotation is also offset to the one given by the curve (i.e. if you want the Y axis of your object to be aligned with the curve’s direction, it must be, in rest, non-constrained state, aligned with the global Y axis). Here again, clearing your owner’s rotation (AltR) might be useful…

The movement of the owner along the target curve/path might be controlled in two different ways:

The most simple is to define, in the Curve and Surface panel of the curve (Editing context, F9), the number of frames of the movement via the numeric field Path Len, and its start frame via the constraint’s Offset option (by default, start frame: 1 [= offset of 0)], duration: 100).
The second way – much more precise and powerful – is to define a Speed Ipo curve for the path (Path Ipo curves, in the Ipo Curve window, ⇧ ShiftF6). The start position along the path will correspond to an Ipo value of 0.0, and the end position, to an Ipo value of 1.0. You can therefore control the start frame, the speed of the movement, the end frame, and even force your object to go forth and back along the path! See the animation chapter to learn more about Ipo curves.
If you don’t want your owner to move along the path, you can give to the target curve a flat Speed Ipo (its value will control the position of the owner along the path).

Follow Path is another constraint that works well with the Locked Track one. One example is a flying camera on a path. To control the camera’s roll angle, you can use a Locked Track and a target object to specify the up direction, as the camera flies along the path.

Follow Path and Clamp To

Do not confuse these two constraints. Both of them constraint the location of their owner along a curve, but Follow Path is an “animation-only” constraint, inasmuch that the position of the owner along the curve is determined by the time (i.e. current frame), whereas Clamp To determines the position of its owner along the curve using one of its location properties’ values.

Options

The Follow Path constraint panel.

Targets: This constraint uses one target, which must be a curve object, and is not functional (red state) when it has none.

Offset: The number of frames to offset from the “animation” defined by the path (by default, from frame 1).

CurveFollow

If this option is not activated, the owner’s rotation isn’t modified by the curve; otherwise, it’s affected depending on the following options:

Fw: The axis of the object that has to be aligned with the forward direction of the path (i.e. tangent to the curve at the owner’s position).
Up: The axis of the object that has to be aligned (as much as possible) with the world Z axis.

In fact, with this option activated, the behavior of the owner shares some properties with the one caused by a Locked Track constraint, with the path as “axle”, and the world Z axis as “magnet”…

Examples

In the following examples, the Path curve has its default “animation” settings (i.e. it linearly goes from start point at frame 1, to end point at frame 101).

Basic

Frame 1.

Frame 51.

Frame 101.

In this first version, the Owner empty has a (0.0, 0.0, 0.0) location, and the Curve Follow option is disabled. As you can see, the owner simply moves along the curve, without any rotation.

With location offset

Frame 1.

Frame 51.

Frame 101.

Here, the Owner empty has a (1.0, 1.0, 0.0) location, and hence is offset of one BU along the X and Y axes, from its “base” position along the curve.