What exactly are the specific differences between a normal HDB flat and an Executive Condo?

An Executive Condo (EC) is buit by private developers, and has comon facilities like a pool, gym, tennis court, etc.

An EC has a price point that is lower than a full blown private property, but stil much higher than an HDB flat (ECs were at about $937 psf as of last month, HDB flats were at $441 psf, while condos were at $1,500+ psf).

ECs count as HDB flats for the first 10 years, so you need to meet all the usual eligbility requirements. After the 10 years, they are fully priviatised and no different from a regular condo.