the kits have an extract thats been flavoured by hops.
extract brewing is when you buy the extract either dry dme, or liquid lme in tins very similar to the kit tins, just without any hop flavouring.
you can then add your own hop flavouring to taste to the extract and brew in a similar fashion to a kit. hops can be added by making a hop tea or adding hop oils,
extract brewing also benefits from a boil of the extract solution to achieve the hot break, this can be done at fairly high concentrations in a 8-10l stock pot,
the bigger the pot the better as at higher concentrations its easier to burn or scorch the wort, then add the prepared concentrated hopped solution to cold water in a sterile fv and wait till its at pitching temps..
it allows you to brew different beers by manipulating the hop additions without all the AG kit
my 2ps worth is the kits on the shelf today allow you to brew beers 'as good as' an avarage commercial pint.. with ag beers you can generally brew beers that you will prefer to a commercial job, however the investment of labour time and enthusiasm may colour your palate
so wait till your pals comment on how much better than a pub pint yours are before you start believing it