An aquarium is a glass-tank filled up with water for keeping soft-water or seawater fishes. Some people use their aquarium for keeping turtles or aquatic plants.
The tank is made of 4 or 5 glass panes, solid enough to keep an important volume of water (up to 1000 liters for a standard aquarium).

The first marble aquaria emerged under the Roman Empire. With the introduction of glass panes in 50, the romans enjoyed the scenery (at least from one side) way better than before.
But in china, domesticating gold fish is a practice dating back to 970 before Christ. If the Chinese people were the first to introduce a sort of aquarium, it's not until the 18th century that people became really interested in the keeping of fish in an aquarium. The 1851 Great Exhibition in the United Kingdom, which featured a massive aquarium, really helped fostering people's interest in keeping fish.

An aquarium is not just a space for fishes to swim in peace. The water tank needs to be completed with several items like a filtration system, a heater...to remove wastes and keep the temperature appropriate for the survival of aquatic species.
When you possess an aquarium, you need to realize that your water-filled tank is not a reduced-scale ocean. In “natural” waters, the aquatic forms of life are part of a system in equilibrium (with waters changing naturally, fishes eating other fishes...). In your aquarium, the food chain is disrupted, the water is not oxygenated naturally, and the death of one fish may perturb the stability of the ecosystem. That's why you need to recreate an ecosystem with proper means (giving planktons to the fish, scattering bacteria...).

Aquariums have been shaped in many styles: coffee tables, toilet seats...there's even a Maquarium, an aquarium formed in the shell of a Macintosh computer.
Public aquariums attract many people curious to see how the depths of the oceans look like (and may even see sharks!!).
The biggest in the world is a 23 800m³ aquarium featured in Georgia, Atlanta.