"differential-equation" is referenced in the pages listed below.

Slope fields provide a way to visualize first-order differential equations and get a feel for the family of solutions. Interactive calculus applet.
Calculus: Many differential equations are not solvable except by numerical methods. Some, however, can be solved by separating the x and y's, then integrating. Interactive calculus applet.
Equations in calculus with derivatives in them are called differential equations, and solving them means finding a function that satisfies the equation. Interactive calculus applet.
Many differential equations cannot be solved exactly, so we need a numerical method to sketch a solution. Euler's method is one such technique. Interactive calculus applet.