These are names which are often given to the most important street in a town. The fact that they are names, not definitions, actually answers both your questions: 1. They do represent the same thing. Whether a town names its most important street 'Main Street' or 'High Street' depends on local traditions. 'Main Street' is more common in the US and Ireland, while 'High Street' is more common in Britain. 2. The whole name is 'Main Street' or 'High Street', so yes, you do capitalise the word 'street'.