These are generally considered the strongest of brews, at 8% ABV and higher. While the original distinction may have come from using only the first running of a high barley percentage brew, today these have expanded to include brews made stronger with inclusions of various types of sugars, such as rock and liquid sugars, honey, maple syrup, or adding any of these including extra wort through a later stage during fermentation called sequential dosing. Either way, you end up with stronger brews and with that, more complexity from additional ingredients including often times aged with different types of oak to help smoothing or rounding it out.
Stronger than most but without the oak.
Aging with different types of oak for extra flavor and smoothness.