yes as reef rightly points out henward, read Wasps basic marine guide it is very good (much better than many beginner marine books I have read, I might add!).
But to answer your questions, Salinity is measured in a variety of ways. The two most common ways are using a refractometer or a hydrometer. Hydrometers are cheaper and less accurate than refractometers. With both instruments, we are measuring the specific gravity, (the SG), at 25 degrees.
In a full marine tank with inverts (i.e a reef tank) the SG is maintained around 1.024-1.026 depending on the individual aquarist. Many people run fish only marines at a lower salinity like 1.022.
Brackish is technically anything between salt and freshwater, and brackish fish are typically more accepting of salinity swings. Typically low range brackish is maintained at 1.005SG and high range is about 1.015... but many fish require increasing salinities as they mature, and many brackish fish require (or do best) in full marine salinity water as adults.