There are three entrances to Zion National Park. The South and East entrance both get you into the Main Canyon of Zion National Park and the third entrance is the Kolob Canyon Entrance and is further north off I-15. 

South Entrance:
This is the most popular entrance and home to the visitor center. Here you will enter from Springdale Utah. 

East Entrance:
Gives access to the Main Canyon via the incredible Zion-Mount Carmel Tunnel and scenic switchbacks. The east entrance is growing in popularity as the East Zion area grows. Utilization of the east side of Zion National Park puts you much closer to the other National Parks in the state of Utah. 

Kolob Canyons:
This gives access to a different area of the park, the two do not connect. This entrance highlights less crowds, a visitor center, hikes, and beautiful 5 mile scenic drive.