Glass or doors in shower areas gets dirty over time from soap scum build up or mineral deposits left over from hard water.
Soap particles and minerals deposits are trapped in the tiny droplets of water that's left over after showering. When that water evaporates it leaves soap particles and minerals on doors and even walls which builds up over time.
Removal is based on the type of glass and the severity of the build up. An assessment will have to be made on whether or not removal is possible.
Maintenance is rather easy; dry up any left over water after showering, this must become habitual in order to achieve best results but keep in mind that this will only slow down build up not eliminate it but it will assist in keeping the Beauty of your Glass or doors.