Only the German army fought on the western front. It was quite short, from the Franco-Swiss border to the English Channel, so in 1915-1917 the front line remained almost unchanged, despite a number of major battles (Verdun, Somme and others).
The German army on the western front occupied Brussels in August 1914 and held it until the end of 1918, and created a threat to Paris until July 1918.
On the western front, the Russian expeditionary corps, the Americans (from 1918), the Portuguese, and the remnants of the Belgian army fought. From the autumn of 1916, tanks were used on it.
The Eastern Front was much longer, from the Baltic to Bukovina in 1914-1916, and from August 1916 it became much longer due to the entry into the war of Romania. In fact, it was divided into two fleets (in the Baltic and the Black Sea) and four fronts (from autumn 1916 to the end of 1917): North, West, South-West and Romanian. Only the Russian army fought there (until August 1916), and then the rather weak Romanian army joined it (in December 1917 the Romanians went over to the side of Germany, a rare case of changing an ally in the First World War).
Tanks were not used, the density of aviation and armored vehicles was lower.
Not only the Germans, but also the Austro-Hungarians fought against the Russian army, they were often defeated. The battles in 1914-1917 were quite maneuverable, the terrain allowed, cavalry was actively used.