Pluralism is just an idea that there are different groups of folks living in a society - instead of a "melting pot," it is more like a "stew pot" with potatoes and carrots and meat all in the same country.
Assimilation is the loss of a personal identity within a society to "blend in" with another society. It is captured rather nicely with the old saying, "When in Rome, do as the Romans." This can be either forced on a group or it can be done voluntarily.
Segregation is just the separation of groups or individuals by a society. This can be backed by laws (THOSE people have to live over in that area), or by the population (we won't let those people live over here).
