These words can be synonyms but I've selected examples where you couldn't really substitute one for another. There are obviously many uses of these words - it's impossible to mention them all! "We need to boost the number of people taking medicine as a degree" (i.e. we need to increase the number of people) "The aim is to bolster weaker industries in the face of economic problems" (i.e. we need to give support to, strengthen) "I am going to reinforce the seams of the bag in case they tear" (can also be used figuratively e.g. "I will reinforce the message the people must not use this entrance") (i.e. make stronger something that is already there)