The original Diablo II campaign contained four acts before its expansion. Each act follows a different region of Sanctuary and ends with a major demon encounter or story milestone.
Act I begins around the Rogue Encampment and leads to Andariel. Act II takes place in the desert near Lut Gholein and ends with the defeat of Duriel. Act III moves through the jungles and city of Kurast, culminating in Mephisto. Act IV is set in the Pandemonium Fortress and concludes with the battle against Diablo.
Diablo II: Lord of Destruction added a fifth act, set in the snowy northern lands around Mount Arreat. That act ends with the confrontation against Baal and became part of the standard experience many players remember.
This distinction matters because later versions, including Diablo II: Resurrected, include the expansion content, making the campaign appear to have five acts. The base 2000 release, however, had four acts; the fifth was expansion content released in 2001.