“Bitter” is a term for an English Pale Ale. Bitters range from low to high alcohol content, and golden to dark amber hue. The bitters have different names depending on strength from lowest to strongest are:
Session or ordinary bitter
Best or regular bitter
Premium or strong bitter
The strongest bitter is what ESB is. The term “Extra Special Bitter” is actually a trademarked name from the Fullers Brewery for it’s strong bitter. ESB beer it has everything I like. It has a lot of flavor layers without trying overly hard for it. ESB is malty and bitter at the same time in a perfect union. The has a deep color the head is small, and carbonation is low. All the factors in this style work very well together. Redhook Brewery does a very good job with the English ESB style beer. I also like the fact that Red Hook’s flagship beer is such an uncommon style in the states. Try this beer, it isn’t for everybody but that shouldn’t stop you from doing it one time at least and you never know you might like it pretty good.
48degrees 
I like Red Hook’s ESB, for a long time I thought they were the only people who made an ESB. Breckenridge Brewery (in Colorado) has a really good one but they only release it in small batches. My brother in law introduced me to them, wish I could find their stuff out here.
The ESB to compare everything else to is Fullers from England. It is a great English style beer and shows how that style has been made for generations. I think it is good to try the old styles of beer and compare to the American versions of the exact same style. American styles have a lot more going on, sometimes it’s good sometimes not. It is nice knowing the old as well as the new.