What's the difference between tempo and rhythm?

Tempo and rhythm are terms that are often mixed up. They are both related to how fast or slow a piece of music is, but in very different ways. Simply put, 'tempo' is how fast you are going; 'rhythm' is how long the individual notes are, relative to each other. Imagine you're driving a car down a motorway. You're driving your car at a certain speed, and there are various stationary things along the side of the road. The car's speed represents tempo - it's simply how fast you're going. The rhythm here is the distance between the things on the side of the road.

(Diagram with three objects; distance between first two is double distance between last two)

So if the car is going very fast - if we have a fast tempo - the car will reach these things objects quickly. But it will still take twice as long to get from A to B as it will from B to C. A slow tempo will mean it takes longer to reach each object - but A to B is still twice as long as B to C. This represents the same rhythm played at different tempi (speeds). We might represent the rhythm of the car's journey from A to C as a minim and two crotchets, or as a crotchet and two quavers - each note represents the arrival at an object. The second two notes therefore need to be half the duration of the first one - but remember that we can then play this at any speed!