Stout should be dark chocolate in colour to jet black.   It may have the following flavours; roast malt, toasty, bitter malt, coffee & choclolate.   It is made with crystal, black malt and unmalted roasted barley to give it these flavours & colours.

Despite the colour and flavour intensity of stout, it still shares 90% of its ingredients with ordinary bitter.   Water with a high alkalinity has mostly pale malt mashed into it with a small addition of darker malts such as chocolate and black.   The more roasted, darker malts neutralise the high alkalinity of the water better than pale malt, producing a better end product.   The intensely roasted unmalted barley gives the stout a malty bitterness and plenty of body as the sugars derived from it are unfermentable.

Coffee may be added to some speciality stouts but a skillful brewer can make a stout taste of coffee (or indeed chocolate) with different malts alone.

Irish stouts are traditionally dry and bitter, whereas English stouts are sweeter and less harshly bitter.   Milk stout contains unfermentable lactose sugars which gives a full bodied, sweet taste.   Foreign & Imperial Stouts are stronger in alcohol and will usually have this balanced with more body and bitterness.   Oyster Stout may contain finings derived from oyster shells or may just be recommended for accompanying seafood.

So what's the difference between porter & stout?

There is much overlap in styles but stout contains unmalted barley and porter doesn't.

History

Dublin and London produced darker beers due to the high alkalinity of the natural waters there.   The higher acidity of darker roasted malts helped neutralise the alkalinity.   The unmalted roast barley comes from the tax on malt for brewing imposed by the English in the 19th century.   Unmalted barley was cheaper to pad the beer out with.   Now its an essential ingredient of course to the stout style and flavour.

Stout has more calories than lager?

Wrong - mostly.   Most of the energy from beer comes from the alcohol stregnth as ethanol and fat have similar calorific values.   The thick body of stout comes from unfermentable sugars with a lower energy rating.   So alcoholic strength determines how calorific a beer is generally.

Stout is a good source of iron?

Absolute tosh!   Iron creates a serious off-flavour in beer and brewers go to serious lengths to eliminate it from their product.   Stout used to be given to hospital patients years ago as a good psychological boost reminding patients of home comforts.   It was an easy way of getting calories into patients with little appetite and helped then forget that they were stuck in a ward.   Try getting a prescription for that today though!