The words packed, pact sound the same but have different meanings and spellings. Why do packed, pact sound the same even though they are completely different words?
The answer is simple: packed, pact are homophones of the English language.
Crowded to capacity: a packed theater.
Compressed: ground covered with wet, heavily packed leaves.
Informal Filled with. Often used in combination: a thrill-packed television series.
A formal agreement, such as one between nations; a treaty.
A compact; a bargain.
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").