Hours of R.A. increase in value from west to east and are numbered 0 through 23. The zenith is the point on the celestial sphere directly above the observer's head. One due south of the observer and the other one due north. Thus the meridian is a plane perpendicular to the surface of the Earth and includes the observer and the Earth's axis. All observers located along the same longitude share the same LST, while the LST is different for observers at other longitudes because of their differing geometry with respect to the Earth and stars.
Civil time is simply the local clock time.
Most political regions of the world support the idea of time zones wherein all residents set their clocks to the same standard time. For instance the time zones used in the continental United States are Eastern, Central, Mountain, and Pacific. UTC uses a 24-hour clock so the notion of a.m. and p.m. is irrelevant. JDN is simply the number of days that have elapsed since the day starting at noon GMT on 1 January 4713 B.C., which was Julian Day 0. Julian Days are useful in a number of ways.