There is a bukkit plugin you can get for a bukkit enabled server that can enable certain chunks to always be active so things like wheat will grow and the like. On a normal server however the world is only active around the players when they are logged in (and where they are logged in)

In general, chunks only 'update' when they are loaded, and they are only loaded when a player is in that chunk.

However, there is a bukkit plugin called 'Forever A-load' you can get for a bukkit enabled server that can enable certain chunks to always be active so things like wheat will grow, redstone will always be active and the like.