Here's a map showing salinity from -60° (that's me!) to 70° (further north than New York, which is at 40°) in the western Atlantic Ocean. The average salinity in the ocean is 35. The units of salinity are a little complicated - some people use what's called the practical salinity unit, or psu, and some people use per mil (0/00‰), which is like percent except that it's a fraction out of one thousand instead of one hundred.
For now, just concentrate on the colors in the map. Red and yellow represent the high salinities and blue and purple represent low salinities. The big mass of green is NADW. The blue is AABW, and the purple is AAIW. The red area near 30° of latitude is water that flows out from the Mediterranean Sea.
Saltier water is denser, but temperature has a bigger effect on density than salinity does. AABW isn't the saltiest, but it's the coldest and the densest. NADW comes next: it's not as cold, but it's very salty. AAIW is near the same temperature as NADW, but it isn't as salty so it's less dense. The water from the Mediterranean Sea is very salty but also very warm, so it's the least dense of all.
Question: Besides temperature and salinity, what else do you think might make water masses different?