The words fisher, fissure sound the same but have different meanings and spellings. Why do fisher, fissure sound the same even though they are completely different words?
The answer is simple: fisher, fissure are homophones of the English language.
One that fishes, as a person or ship engaged in fishing.
A carnivorous mammal (Martes pennanti) of northern North America, having thick, dark-brown fur. Also called pekan, wejack.
The fur of this animal.
A long narrow opening; a crack or cleft.
The process of splitting or separating; division.
A separation into subgroups or factions; a schism.
Anatomy A normal groove or furrow, as in the liver or brain, that divides an organ into lobes or parts.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").