A machine used for cooking. It uses gas as its fuel. You can stand pans on top of it or put a dish inside it.
OK, this got downvoted, probably for being short and pants, so here's some more. Please /msg me if you downvote in future.
A typical gas cooker has an oven section at the bottom and has hobs at waist height. The oven will have a single control to adjust the temperature, though there will be a a gradient from top to bottom. The provision of shelves, usually with adjustable heights, allows the cook to make use of this feature by placing dishes in different parts of the oven where their contents will cook at different speeds. This can be used to best effect when preparing a meal that consists of different foods, some of which cook faster than others. By positioning them correctly within the oven you get a chance to finish cooking them all at the same time.

The hobs are designed to allow the cook to prepare food in pans by placing them above a heat source. Each hob has its own temperature control. The use of gas allows the temperature to be quickly and finely adjusted, as compared to electric hobs, which tend to be slow. This is because the pan is heated directly by the burning gas.

Some cookers will also have a grill section. This is sometimes found within the oven, but more usefully it may be at eye level. Positioning it at this height makes it convenient for the user because they can then monitor the state of the food as it is grilled. However, it can cause problems for shorter than the average adults and for children. It is usually not recommended for a child to use a cooker without supervision, though this depends on the maturity and experience of the child.

Gas cookers used to be a fairly popular choice for the suicidal. This is because it was relatively easy to turn on the gas supply without igniting it. The user could then place their head in the oven and kill themself by replacing their normal air supply with lungfuls of gas. I may be wrong, but I believe that modern gas supplies are not lethal, by some wonder of chemistry.

A further reason that some people choose not to have gas cookers in their homes is that they can be dangerous if the pilot light goes out. This device is a small outlet for the gas that is intended to remain constantly alight. Then if any gas leaks from the supply it is burnt off before it can collect in dangerous quantities. The pilot light is also a useful feature which allows the user to light the other sections of the cooker without needing matches. However, when it goes out gas can fill the house until some other influence causes it to ignite explosively.