Aggregate fruits, as you can see with the flower diagrams above, develop from blossoms containing more than one simple pistil, and when those pistils mature they stick together to form one thing -- the aggregate fruit. If the term "pistil" throws your for a loop, you may want to review our Standard Blossom page. Above, the pistils are the pale green, oval things.
In the aggregate fruit known as the Blackberry, pictured at the right, each of the Blackberry-blossom's simple pistils has matured into a tiny drupe, or "drupelet," and all the flower's drupelets are adhering into a single unit, an aggregate fruit. Each of those spherical items on the Blackberry "fruit's" surface in the picture is a drupelet, but the the whole blackberry is an aggregate fruit (and not, technically, a berry, which is described on the Simple Fruits page!).
The above picture may explain things a little better. The item in the picture at the far right is an old blackberry flower. After pollination the white petals have fallen off and now the clusters of stamens are turning brown and about to fall off. Notice the granular appearance of the green thing in the these old flower's center. This is a cluster of many separate pistils. Notice the brown, drying-up styles that in the flower rose above the individual ovaries.
Then at the left in the above picture you can see that the individual pistils are enlarging and growing together. This is a half-formed blackberry and the immature drupelets still have the old, dried-up styles atop them. This thing will grow to about twice its size here, turn red, and finally turn black, juicy, and delicious!
Many flowers, such as Buttercups, have several simple pistils that develop into clusters of tiny fruits (achenes and drupelets) but these do not stick together -- don't aggregate into one fruit-like unit. Rather they develop independently and fall to the ground separately. Thus they are not aggregate fruits. Again, aggregate fruits have two main features: