What is this thing?
A particle generator. It generates particles! And then depending on speed, direction, gravity, air resistance and other variables they fly all over the place, making pretty pictures. Until they die (but they reincarnate). You can tweak their behaviour by entering different values for the different properties.
In those fields?
Yeah, new values take effect as soon as you leave the input field. Clicking in the field and using the up/down arrow keys makes them take effect immediately, this also in/decreases the value by sensible steps, and makes sure the value stays within allowed limits (but for most, you can also manually enter a value outside that range)
What are these values?
Particle count: maximum number of particles there should be. It'll be less at the start, or after a large group dies off (like when decreasing the Life value) but new ones will be created to catch up. You can see the current number in the particle counter on the right of the fields.
Life: how long a particle lives for. If Life is increased, new ones will be created faster for a bit; decreased: excess particles will be removed immediately. A value of 0 makes them immortal (which you have to enter manually).
Gravity: something Newton invented. You can set it to negative values too and see all the particles fly up! Try making them hover in place.
Air resistance: higher values slow down particles more (up- and downwards), a value of 0 equals vacuum. Its force is proportional to the particles' speed squared. Negative values will.. well, that'd be silly, right?..
Bounciness: how much of its speed a particle retains when hitting a wall or the floor. 0 = none (think raw eggs), 1 = all of it. Higher values than 1 are possible.
Initial speed: max speed a new particle can get, but it'll usually be lower. Tried to use a normal distribution, needs looking at.
Autostart: this value only affects urls saved with the 'make url' link.
Cooldown: rate at which it changes colour from white to black (through yellow, orange, red). A higher value makes it fade out faster, but the particle may still bounce around for a while if its Life value is high enough.
Size: a property of the visual representation of the simulator's main entity type.
Trails: a particle's previous position is still visible for a while, higher value = visible for longer. Value of 50 means 'forever'.
FPS: Frames Per Second, how often the next scene will be rendered in a second. Higher is faster obviously, most browsers cap it at about 75. If you have a lot of particles it.. might not be that useful.
X, Y, Width, Height: are the particle source's coordinates, and the width/height of the canvas. X goes from left to right, Y from top to bottom.

You can use cursor keys inside the input fields to change the value with sensible range/steps. Shift-{cursorkey} multiplies the step value by 10.

Shortcuts & links
(Un-)Pause: pauses/restarts the animation, shortcut-key P or spacebar.
Reset values: resets all the input fields to their default value (which can be the url parameter values). Existing particles keep their speed and life until they run out though. Shortcut-key R.
Clear canvas: this restarts the animation with all current values, but zero existing particles and a clean background. Shortcut-key C.
Make url: puts all the current values in a URL for you to copy and give to your pet rabbit.

Oh yeah, if you click in the canvas the particle source starts following the mouse around, until you click again, that'll make it stay there, etc.

PauseUn-Pause, Reset values, Clear canvas, make url