The measurements lists what part you measure at (and inch below the armhole, etc.) The sizes given are a half-size, meaning it only measures a single side of the fabric.
For example: My chest size is a 38", so I wear a Men's Medium. (I could technically fit a small, but a like it a little extra loose)
If you wear a women's size (I don't know you gender, so just defining both), their fitted shirts are more difficult, but still understandable. They curve in at the waist, so instead you'd need to measure the chest, Waist, & hips width. (for comparison, I'd have to wear at least lady's 2X to fit. )
I never really use the shoulder-measurement, since the chest-width is usually the main defining factor, but I guess if you're really buff that could cause some problems. Otherwise, disregard.