Link αi-1 ai-1 di θi type min max
*As in the teaching materials, we are using the modified DH parameters.

Red: X-Axis
Green: Y-Axis
Blue: Z-Axis

Press 'q' to enter IK target mode. Move the target to a desired position, then press 'q' again. The manipulator will then move to the desired position, if applicable.

αi-1: Link twist or offset angle (measured from zi−1 axis to zi about the xi axis, again using a right-hand rule. For most commercial manipulators the offset angles are multiples of 90°)
ai-1: Link length (the shortest distance between zi−1 and zi axes. It is measured as the distance along the direction of xi coordinate frame. For intersecting joint axes the value of ai is zero. It has no meaning for prismatic joints and is set to zero in this case)
di: Link offset (distance from the xi−1 to the xi axis measured along the zi−1 axis. If the joint is prismatic, di is the joint variable. In the case of a revolute joint, it is a constant or zero)
θi: Joint angle (the angle from the xi−1 to the xi axis measured about zi−1 axis. This is defined using a right-hand rule since both xi−1 and xi are perpendicular to zi−1. The direction of rotation is positive if the cross product of xi−1 and xi defines the zi−1 axis. θi is the joint variable if the joint i is revolute. In the case of a prismatic joint it is a constant or zero)
type: Joint type (revolute, prismatic or fixed)
min & max: min and max joint value respectably In degrees for revolute joints

Although an important convention within the field of robotics, the DH approach to generate the FK for manipulators is far from the only option. Another similar approach is to use screw theory.
If you are familiar with 3D graphics and/or scene-graphs, a serial manipulator is nothing more than a set of parent/child relationships, where the rotation or translation of each child is restricted to zero or one DOF. The kinematics of arbitrary manipulators can easily be setup by combining four different transformation matrices. That is, rotation about X, Y, Z and pure translation. Using this approach you are free to assign frames however you want, and you can define more than just the links and joints contributing to the FK. I.e. defining where hydraulic cylinders are attached to the manipulator. Take a knuckle-boom crane for instance. It is simply Rz1) * Tz * Ry2) * Tx * Ry3) * Tx given a right handed coordinate system with z up.

Inverse kinematics (IK) is normally computed using the Jacobian matrix. Taking the inverse of this matrix you can solve for joint velocities given a desired Cartesian velocity. However, regular inverse only works if the matrix is square. This is not the case for many types of manipulators! Luckily, algorithms exists which lets you solve the IK using the Jacobian even though the matrix is not square! A couple of examples is the pseudo-inverse and transpose methods. However, you'll probably want to stick with the excellent Damped Least Squared (DLS) method, which is the method used by this tool. An excellent paper that covers the three previously mentioned methods can be found here.
Computing the Jacobian for an arbitrary manipulator (or any function) is actually pretty straightforward. To compute the (i, k)-th entry of the Jacobian matrix, simply use the the following formula:

where pi(x) gives the i-th component of the position function, x0,k + h is simply x0 with a small delta added to its k-th component, and h is a reasonable small positive value (say 1E-6).

Here, only the position function is considered. Hence, the IK will only be solved with respect to a desired position. However, the formula can easily be extended to include orientation of the end-effector. Simply increase the number of rows in the Jacobian matrix by three and use the position function for the three first rows, and the rotation (euler angles) for the latter. This is only useful if the manipulator is able to rotate the tool in a sufficient manner though. I.e a 6DOF industrial manipulator.