Can you play all the Diablo 2 acts on Diablo 2 Lord of Destruction?

If I have both Diablo 2 and the expansion pack: Diablo 2 : Lord of Destruction installed, can I play the Diablo 2 acts on the expansion pack?

The reason is, I've heard that the expansion pack has a lot of gameplay improvements, if so, I want to play it rather than the original Diablo 2.

The LoD expansion expands the original game - it includes everything in it, but adds more content such as new classes and a new act. It does not replace the existing acts, so the answer to your question is yes, you can play all the D2 acts on D2:LoD.

If you have Lord of Destruction there's not much reason for you to run the regular D2 (unless you want to play online with friends that don't own it, if I recall correctly - it's been a while).