How do chokes work?

[GlasgowTitan]

I remember as a kid my dad's old cars used to have a choke and when starting up in the mornings it had to be pulled out or we be going nowhere. I take it nowadays cars have some sort of automatic choke, or the engines have evolved so that they aren't needed..? I know if I had to drive a car with one I wouldn't do it right

 

[GlasgowTitan]

ok its kinda making sense now, beacause in the mornings Ive noticed that my zetec engine idles at about 1500rpm, then when i get to uni its idling at about 700rpm or something...