Canning first originated during the Napoleonic Wars, and is a method of preserving food. It was initially developed as a means of transporting large quanitites of food without it spoiling; something that was important for an army, as a lack of food would require foraging from the land, which is both slow and dangerous. The first "cans" used were not cans, but glass jars but, not surprisingly, these proved to be a bit fragile when transported.
The tin can was then invented as a method of canning - which is why they are also called tins, although modern-day cans are more commonly made from steel, or possibly aluminium. The food is processed and sealed inside an airtgiht container, the can. Canning is not a way of indefinitely preserving food; the expected shelf life is usually in the region of 1 to 5 years.
The can opener, which is needed to open cans, was not invented until 30 years later. Originally, cans were opened with sharp implements, such as knives.
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