The largest KNOWN nuclear weapon was made by the Soviet Union (Russia) and was called the "Tsar Bomba" (King of Bombs). It had a yield of ~50MT.

Exactly the same thing as a TNT bomb of the same yield.

About as dangerous as conventional weapons of the same yield, plus the radiation effects.

"Dial-a-yield," or Variable Yield, is a method of adjusting the yield of a nuclear weapon through various means. While most modern high-energy weapons are thermonuclear, both fission and thermonuclear weapons can have their yield adjusted. In a boosted fission weapon (which can also be the primary to a staged radiation implosion weapon), the yield can be adjusted by changing the amount of deuterium/tritium gas that is injected into the plutonium pit, or by the timing of the external neutron initiator, or both. In a staged weapon, causing the secondary to not ignite by adjusting the yield of the primary (see above), or blocking the radiation channel in some way, can also change the yield of the weapon.

Basically, a conventional bomb uses a chemical explosive as the source of its destructive power. A nuclear weapon uses nuclear material to create an explosion. A nuclear explosion is much larger, and also emits ionizing radiation. A chemical weapon does not emit any radiation.

A nuclear weapon's yield is measured in Kilotons (thousand tons). In very simplified terms, this means that a nuclear weapon with a 475 kiloton yield produces an explosion comparable to 475,000 tons of TNT (TNT is a chemical explosive). That's A LOT of TNT and it would take up a bit of space. A nuclear weapon with this yield may only be a few feet long and a foot wide, and the actual nuclear material may be the size of a grapefruit.