There is no difference in meaning between the two. The difference and the preference for one over the other will vary based on the context. In the two examples you provided, the second usage is much more common than the first:
Computers have changed the way we interact with each other.
Monkeys are very smart creatures.
While the differences between the two are extremely subtle and the clues are probably found in the part of the discussion we don't see, I think it all relates to the fundamental rule about using an article to refer to something specific, and not using an article to refer to something that's non-specific. In this case, you need to think in terms of a shade of "specific", since these statements are intended to represent a generalization. For example:
The monkey is a very smart creature.
Using the article puts emphasis on the fact that there is a category called creatures, and that the monkey is a specific member of the category. Creature is more than just a word; it's a category.
Monkeys are very smart creatures.
There is no intended or implied reference to a category of beings called "creatures." Creatures is a word, not a specific category. Monkeys means "monkeys in general" without implying that they are members of a broader category.
The computer has changed the way we interact with each other.
The computer is probably a reference to the personal computer as opposed to business mainframes and supercomputers. So by using the article, you're referencing a subset of all computers (the computers people have used that actually have affected how they interact with each other).
Computers have changed the way we interact with each other.
Computers means computers in general. There is no specificity intended or implied.