From Diablo Wiki
Expansion Packs refer to additional games added onto Diablo III, to expand and extend the story and gameplay. Diablo II had only one expansion, Lord of Destruction, while the Diablo III developers are planning at least two for Diablo III.
Diablo III is not being released in multiple parts, ala Starcraft 2, but the team has made if fairly clear from the early days that they have a lot of game plot and content and plans to work into Diablo III; enough to fill out at least one expansion pack.
 Expansion Commentary
- in.Diablo.d3: WoW and SC2 have lots of expansion. D3? One game with expansions, or multiple parts of the game?
- Jay Wilson: *hesitates over his answer* Right now we’re just focused on getting the game done. We have ideas, but we don’t know. This one’s a handful. All I’ll say is that the previous Blizzard games just did one expansion, like Starcraft and Diablo II, seemed like a missed opportunity. We’ll make more D3 content until we feel like we don’t have anything more to say.
- in.Diablo.d3: Are there plans to make more regular expansions? Each 18 months?
- Jay Wilson: haven’t really thought about it yet. All of our time/energy to make D3.
 Post-Release Content
Asked at Blizzcon 2010 about releasing more content in the form of patches or expansion packs, Jay Wilson explained their post-release strategy is to focus on major updates via expansion packs, rather than regular small updates via patches.