There's more rubbish written (in the media mainly) about groundbait than about almost any other product we chuck in the water - second only to flavouring maggots.
Groundbait can be broken down into three broad categories:
Traditional is your standard brown or white crumb - bread based and been in use (in the UK anyway) since the days of old Izaak - still useful for bulk feeding or to water down other more expensive products.
Continental groundbaits are mainly used when targeting silver fish - Van Den Eynde, Sensas etc - they started to become popular in the mid/late 80s - coinciding with the increased popularity of pole fishing and England's dominance of the world championships. Generally biscuit based but with many other ingredients added to alter the flavour, texture or how they reacted once in the water. VDE Special for instance is a very light cloud bait designed for fishing on or near the surface - at the other extreme VDE Turbo is a heavy groundbait designed to hit the bottom of deep lakes or rivers and remain intact until it gets there. Supercup is a personal favourite - it has an aroma of chocolate and I rate it highly for skimmers and bream. The long since vanished Picardie smelt strongly of garlic/curry and was the groundbait of choice when targeting gudgeon. The various products could be mixed together to produce a groundbait with different qualities - Supercup alone is quite light and not really suitable for balling in - mix it with VDE Secret and you get a sticky mix that'll hold plenty of particles and get to the bottom reasonably intact. Unfortunately 'cos the word carp doesn't appear on many of the bags they aren't as popular as they once were and are quite difficult to find in many tackle shops.
Carp groundbaits - ignore the hype - they're all fishmeal based. Sillybait was the first in the late 90s - must of been about 95% fishmeal - was a nightmare to mix 'cos fishmeal alone doesn't bind well so needed the addition of brown crumb. A lot was used in conjunction with Magic Meat Powder (icing sugar in a fancy bag) which made your little cubes of meat sticky so when sprinkled in Sillybait enough adhered to make a soft, fishmeal flavoured bait - soft pellets or expanders hadn't yet been invented. Until its use in animal feed was halted after BSE it was possible to buy a 25kg of fishmeal for under a tenner from any animal foodstuffs supplier - and it was practically identical to Sillybait, a quick sieve to get rid of the bigger lumps was all that was needed.
Modern carp groundbaits add a different colour, perhaps increase the sugar content to produce a 'sweet fishmeal' but the actual differences are minimal. Some advertise that they're made from crushed pellets - pellets that are 35-40% fishmeal to start with - Sonubaits, Dynamite (swim stim), Ringers, Bait-Tech, Sensas Crazybait - the list is endless. My personal favourite is the original Crazybait - by it's smell and texture I think it has a higher proportion of fishmeal which is what I want in a groundbait for carp but pick one and learn how it works - how much water it absorbs and how long it takes to absorb it all - no good mixing a groundbait that takes an hour to become fully wetted, 20 minutes before you want to use it.
And my personal bugbear - never sieve dry groundbait - you've bought a bag of groundbait that has been painstakingly mixed and blended to achieve a certain texture and action once in the water - it may, for instance, have crushed hemp in it which helps to push bait out of a feeder or break down balls. Sieve out all the bigger particles and you completely alter the groundbait - if you don't want large particles in your groundbait buy a bag that doesn't contain them in the first place. Groundbait is expensive enough as it is without chucking half of it away.
And one thing is certainly true - fancy groundbaits have caught many more anglers than they ever have fish.

Personally I use a lot of this
Boland - cheap but just as good as the stuff costing three times as much - postage is a bit steep but fortunately the UK importer only lives a couple of miles away so I can just nip over and grab a bag when I'm passing
Simon