Photoperiods are plants which will only go into flower when the light source has been reduced to 12hours of light and 12 hours of dark. This can be manipulated indoors but outdoors is completely dependent on nature. Indoors, the plants can be manipulated in many different ways to obtain a large number of bud sites and there is no time limit for when you actually HAVE to put the plant into flower... it can stay in veg for a very long time - and can even be put BACK into veg after it has flowered if so desired... there's a LOT of "play" that can be done with photoperiods.
Autoflowers are what the name implies... their cycles have been pre-determined and the grower has no say in it.... they will generally go into pre-flower at about week 3 or 4 and into flower a week later. They do not take well to major stress (any of the manipulations that you can do to photoperiods such as topping, fimming, even transplanting unless done VERY carefully) as plants need to have time to recover from this stress and autoflowers often don't HAVE the time... but if you're looking for a fast harvest and don't want to get too fancy with things, they are fun and rewarding. The harvests will never equal the size of a harvest from a well-grown photoperiod - but they're fast and decent. You can have multiple harvests growing autoflowers throughout the year (each plant will only give you one harvest but you can have multiple grows) whereas you generally only get one harvest from photoperiods a year (at least the ones grown outside).
Cheers!