Berries are a feature of goat evolution, and once unlocked through the Upgrade shop a berry tree is delivered to the Farm level periodically, as shown by the bar at the bottom of the screen. Further upgrades can be purchased to decrease deliver time, and increase the time the berry effect lasts. There is also an achievement for mutating a goat under the effects of a berry. There are three different types of berries, as listed below:
Blue berries cause a goat to turn blue in color and shake violently before pooping a diamond. As the effect time is increased the goat can poop more than 1 diamond. The highest berry upgrade will allow the goat to poop a total of 3 diamonds if it's not interrupted by mutations.
Red berries cause a goat to turn red in color and produce a lot of poop in a very small space of time.
The bigger the goat, the bigger the poop (and more money). The ultimate being cannot poop stars.
Yellow Berries cause a goat to turn yellow in colour to emit a shower of money each time it gets mutated.
The yellow berry is a factor of your coins/sec. The most useful way to use the berry is by allowing the smallest tier creature to eat the berry, and quickly mutating it as many times as possible in the given time. This will give you a permanent boost in the number of coins produced in a second.
In addition, every time the goat mutates, you receive a coin boost, which seems to grow the further you progress in the game.