Replacing mountain bike bars & stem with drop bars
There are four measurements you need to be concerned with. The two most critical are the fork steerer diameter and the bar clamp diameter. 

The fork steerer diameter is normally referred to as 1", 1-1/8", or in some cases 1-1/2". The bar clamp diameter can be 1"/25.4mm (typically MTB), 26mm (typically road), 1-1/8"/31.8mm (commonly called oversized or OS). 

The other dimensions are the length of the stem (normally the distance between center of steerer clamp and the center of the bar clamp) and the angle, which is usually stated as degrees from perpendicular. Most stems can be mounted pointed up or down but check the installation sheet first. If two stems are 120mm long but one is +7 degrees and the other is +34 degrees, the 34 degree stem will make the bars feel higher and closer.