Board games are a very, very broad term.
For coparsion, let's compare Ludo, Monopoly, Kalaha and so forth to "casual" games.
Many people are familiar with light and friendly board games like settlers, smallworld, carcassonne, frag, zombies, ave caesar, power grid, eufrat & tigris and the like. These typically take 10-15 minutes to explain to a new player, and a game can be played in an hour or two. Following the previous classification for "casual" board games, these are the wiiware/xbox live arcade games. Anyone can pick them up and enjoy them, even if they aren't (board) gamers per se.
After that, you have slightly more complex games. Things like Le Havre, Puerto Rico, History of the World, Arkham Horror, Space Hulk, Doom or Betrayal at the House on the Hill will easily take 3-4 hours to explain, set up and play for new gamers. These are your "console" board games. They have a fairly broad appeal, you can learn them with a little time investment, and they can keep you entertained for ages.
If you have the time to learn them, games like Twillight Imperium, Battlelore, Android, StarCraft (Yes, they made a board game of it), Battle for Armageddon, Through the Ages, Memoir '44, Junta (Viva el Presidente!), Titan (We once had a game stretch to 14 hours), Britannia or Shogun will easily take a full evening of your time. These are the board game equivalents of old-school PC games. You'll have to keep that poster-sized map handy, and the reference booklet as well as the manual.
But if you have a truly impressive beard and still remember wether your AdLib or SoundBlaster used IRQ 5 or 7, there are the Old-Schol "hardcore" boardgames. We're talking things ike Avalon Hills Advanced Squad Leader. Diplomacy. Empires in Arms (best played 2 weekends a month over the course of 9-10 months to finish one game). Gettysburg and many others. These are the games in which hundres upon hundreds of little cardboard tokens battle for supremacy on hex grids. Where rulebooks come in several volumes, and grognards thoughtfully consider the effects of encumberance when deciding their next move.