HDTV broadcast systems are defined by:
The figures 720 or 1080 respectively each indicate the number of vertical pixels; or, in other words, the number of horizontal lines.
Here you can find more details about interlaced and progressive scan >>>...
Since the indications fps always refers to “frames per second”, the figure after the forward slash incdicates the number of frames per second. Sometimes there are indications without a slash, for instance 1080i50. In this case the figure 50 does not represent for the number of full frames but rather the number of half frames (i.e. 1080i50 = 1080i/25).
Why is that number lower with interlaced than with progressive scan?
Example: 1080i/25 or 1080p/50 respectively
With the interlaced scan two fields are subsequently transmitted. Therefore, within the stipulated time period of one second, a maximum of 50 half frames can be transmitted. This equals 25 full frames.
In contrast, with progressive scan every image is transmitted as a full frame. Hence, a maximum of 50 full frames can be transmitted per second.