detergent

1) First of all, ammonia is a weak alikali that upon dissolving in water (ammonia gas is very soluble in water), it hydrolyses to give ammonium (NH4+) and hydroxide (OH-) ions:
NH3(aq) + H2O(l) <---> NH4+(aq) + OH-(aq)
And then the hydroxide ions can decompose those grease and oil on the windows by a process called saponification:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/Saponification.jpg

Therefore this enhances the ease of removal of those oil and greases. Moreover, the carboxylate ion (which is in fact a soap molecule) can help to further remove those grease and oil by means of emulsification.
2) First of all, sodium hydroxide does not react with glass which can be proved from the fact that sodium hydroxide reagent can be stored in a glass reagent bottle.
Instead, ammonia solution is preferred rather than sodium hydroxide because of the potential danger from sodium hydroxide. Since sodium hydroxide is a strong alkali while ammonia is weak, the hazards caused to human skins will be less for ammonia solution when accidentally spilt on human bodies.

1.Because ammonium solution is a alkaline solution, it will react with the dirt on the glass and clean it
2.Because sodium hydroxide is a strong alkaline it will destroy the glass but ammonium is weak alkaline