They look like a log cabin. You can buy tickets there, get info about bus schedules as well as get a shelter from the weather.
Below is bus stations map of Plitvice Lakes.
Bus stop on the cloudy and rainy day. As this photo is taken in the autumn’s dusk, there are just a few people waiting for a bus to Zagreb. This is a bus stop near entrance 2.
Bus stop near Entrance 1 with a bridge to access both sides of the road and the Park.
Bus Stop 1 : 44.906377141,15.611395092 / 44°54’23.0″N 15°36’41.0″E
Bus Stop 2 : 44.904945403, 15.612636644 / 44°54’17.8″N 15°36’45.5″E
Bus Stop 3 : 44.882401, 15.623363 / 44°52’56.6″N 15°37’24.1″E