Stouts tend to have a fairly high roasted malt flavour. And while some dark ales are made with roasted malts, they tend to be much lighter than their stout counterparts.
Porters are similar to stouts but are much less dry and even less heavy-handed when it comes to roasted malt flavour. They’re also much sweeter in general. Dark ales, while on the sweeter side are much maltier and full flavoured than the much lighter porter.