Technically speaking, Zemun is one of Belgrade’s municipalities.
We who were born and raised in it, though, don’t give a hoot about administrative definitions: Zemun is a town near Belgrade.
All joking aside, Zemun does have a distinct spirit quite different from Belgrade. Since it was never under the Ottoman reign, the architecture is visibly different, typical for Central European structure, dissimilar to the street maze you come upon in Belgrade.
Zemun lies on three hills, Ćukovac, Kalvarija and Gardoš, which makes biking almost impossible but offers excellent viewpoints, the most famous of which is Gardoš.
Looking from Kalemegdan towards the approaching Danube, you cannot help but notice a reddish tower on a hill (this photo is taken from Belgrade quay). It was built by Hungarians in 1896, for a millennial of their life in Pannonia and named after Hunyadi János, a Hungarian hero who fought against the Turkish invasion and died there.
Gardoš used to be a part of town where fishermen lived and even today you can see nets, boats and equipment along its steep cobbled streets. The tower (finally open after a massive reconstruction!), next to which there is also a restaurant Gardoš, offers an almost equally amazing view of Zemun, the confluence, Belgrade, and beyond.