Sekt is the German name for sparkling wine. It’s not a region like Champagne, nor is it restricted to a method of production. Historically, the view of Sekt has been negative but as with most formerly poor wine regions/methods/approaches, quality is definitely on the up.
Bottles labelled simply ‘Sekt’ can be made with grapes from anywhere in the European Union, where as 'Deutscher Sekt’ indicates the base wine has to be German. The 'b.A’ addition to the name, indicates a level of quality greater with only certain grape growing regions allowed. The 'Winzersekt’ label means the wine is single-estate, single-variety wines and often reserved for the highest quality wines.
Sekt can be made either in tank method (like Prosecco) or the traditional method (like English Sparkling and Champagne).
Given the variability in labelling terms, it’s useful sometimes to have a Google of the producer in question to find out what their method of production is and where the grapes are from.
All in all, if you do some research there’s some very good value to be found in Sekt. The vast majority of it doesn’t leave Germany, which also helps when buying in the UK.