We group similar plants into groups called families. "Nightshade" is the common name for a huge family of plants (Solanaceae). They're all flowering plants, many with edible fruits. They share a few similarities in the shapes of parts of their flower, but otherwise they are dramatically different in size (tiny shrubs to trees), shape (vines, bushes, ...), color (whole rainbow), fruits, etc.

Some common edible nightshades: tomatoes, potatoes, eggplants, bell peppers, and chili peppers.

There are also nightshades that are poisonous: belladonna, tobacco, mandrake, henbane, etc.

There's no reason to avoid the edible nightshades unless you have an allergy specific to the plant. Obviously, you shouldn't eat the poisonous ones.