Primula are the poster children for the pin and thrum phenomena, although they are not the only ones that employ such a way of pollination. The physical differentiation between the two types of sexual (ahem...) positioning within the same species of flower is easy to understand:
The "pin" form is when the pistil (the female part) within the flower is much longer than the stamens (the male parts). So in these photos, the stamens are much shorter than the pistil (otherwise known as the "pin".) These are pin flowers.
Then there is the "thrum" form, when the male and female parts switch positions: stamens are relatively long, and the pistil is short (and hardly detectable in the photos). These are thrum flowers.
These two flower forms are uniform on individual plants. That is, all flowers on a single plant are either pin flowers or thrum flowers. A single plant can never produce both kinds. Species that employ this dimorphic flowering are insect pollinated. The purpose of this rather ingenious evolutionary tactic is to insure cross pollination. Pin plants can pollinate thrum plants and vice versa, but pin plants do not pollinate other pin plants (or themselves), and thrum plants do not pollinate other thrum plants, nor do they self pollinate.
I learned about this long before the age of computers, and while the basic "what" was easy to understand, no one could tell me the underlying reason "why". Why does it insure cross pollination? Everywhere I read that these two kinds of flowers force cross pollination, but nowhere could I find out how! ???
Well, I finally did figure it out for myself, and I have to say I was quite proud of my deductive thinking when I did. So much so that, if you don't know, I'm not going to say ;), at least right away, and let you ponder for a while. Someone else can do the explaining.
Meanwhile, for those who already "know it all", there is a good chance you don't ;D. Wikipedia goes into more detail with correct terminology and the concept of trimorphism, too. Read it here:http://en.wikipedia.org/wiki/Heterostyly