Straight is primarily (1) an adjective meaning extending in the same direction without curving, and (2) an adverb meaning directly. (It does have some rare noun senses, mainly referring to straight parts of roads and straight lines.)
Strait is almost always a noun. It means a narrow channel joining two larger bodies of water. It also has a few mostly archaic adjective definitions, but these are almost never used.
Karen will run straight back to her room. [Washington Post]
Stratford Constable Nicola Howells said investigations were continuing into the crash that occurred on a straight road. [Stuff.co.nz]
A near straight line around the apple from stem end at the top to calyx end at the bottom. [Yakima Herald-Republic]
From 1830-1925, there were 137 recorded tragedies where ships were lost at the west entrance of the strait. [Mukilteo Beacon]
An iconic landmark helps protect most low-lying Bay Area communities from a devastating tsunami: the Golden Gate strait. [San Jose Mercury News]