The words straight, strait sound the same but have different meanings and spellings. Why do straight, strait sound the same even though they are completely different words?
The answer is simple: straight, strait are homophones of the English language.
Extending continuously in the same direction without curving: a straight line.
Having no waves or bends: straight hair.
Not bent or bowed; rigid or erect: a straight, strong back.
A straight form or position.
A narrow channel joining two larger bodies of water. Often used in the plural with a singular verb.
A position of difficulty, perplexity, distress, or need. Often used in the plural: in desperate straits.
Difficult; stressful.
Having or marked by limited funds or resources.
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").