TL;DR You need to look at both entries when you are looking for information about the city, which is called Marburg an der Lahn nowadays. The problem of that website you are using is, that they do not use the most recent borders of Marburg.
Unfortunately on the website you are using you will not be able to find everything belonging to Marburg an der Lahn in its borders today in one menu entry. This is due to the fact that churches in Germany have their own administrative districts.
The entry Marburg Stadt probably refers to the core region of the town Marburg an der Lahn. This is in German it is also referred to as Kernstadt, or Innenstadtbezirke. It is quite possible that you are looking mainly for that. (On the map below this is the part which is enclosed in the bold dotted line.)
Marburg Land on the other hand refers to the outer townships and some other communes as well.
Some entries there belong to the town Marburg today, prominent examples are Cappel, Moischt, Wehrda, etc.. These are referred to as Außenstadtteile or Außenstadtbezirke. (On the map below this is the yellow area without the above.)
Then others in the list are communes by themselves, like Cölbe. And then there are entries that belong to other communal districts like Ebsdorf belongs to Ebsdorfer Grund, Caldern belongs to Lahntal, etc..
The a map of the townships of Marburg an der Lahn, which is the district capital of Marburg-Biedenkopf, is pictured below.
Source: Lencer, Wikimedia Commons File:Karte Marburg Stadtteile und Nachbargemeinden.png.
Prior to 1974 Marburg was a town without a district, which might well be the entry Marburg Stadt. There was also a district called Marburg (Lahn) which included cities like Kirchhain (which has its own entry) and Amöneburg (which you will find under the entry Kirchhain) and others (see below). Parts of the district belong now to the town Marburg an der Lahn, while others were includes in different communal districts and towns.
Depending on what you are looking for, you might want to look at entries from Marburg Land, too.
Source: Gesetz zur Neugliederung der Landkreise Biedenkopf und Marburg und der Stadt Marburg
§ 22 LandkrMNglG – Landkreis Marburg-Biedenkopf
(1) 1Der Landkreis Biedenkopf mit den Städten Biedenkopf, Gladenbach und den Gemeinden Angelburg, Breidenbach, Dautphetal, Bad Endbach, Steffenberg und der Landkreis Marburg mit den Städten Stadtallendorf, Amöneburg, Kirchhain, Neustadt (Hessen), Rauschenberg, Wetter (Hessen) und den Gemeinden Cölbe, Ebsdorfergrund, Fronhausen, Lahntal, Lohra, Münchhausen, Weimar und Wohratal werden zu einem Landkreis mit dem Namen "Landkreis Marburg-Biedenkopf" zusammengeschlossen. 2Sitz der Kreisverwaltung ist die Stadt Marburg.
(3) Die Stadt Marburg wird in den Landkreis Marburg-Biedenkopf eingegliedert.