They're random research chemicals, generally stimulants or stimulant/empathogens, and they're often cut to hell and back, but may not be.
Often drugs that are cut are poorly mixed, and result in some samples being stronger than others (esp with something like MDPV, which is probably cut... what, 5:1 or more?).
Different batches of the same product may be completely different. Successful products of one name are often copied in name (but not contents) by other companies, so Joe Headshop's "Snow Leopard" may be different from Sue Smartshop's "Snow Leopard".
Stay the hell away from branded legal highs. If they won't tell you what's in them, don't buy them. Most of them are garbage (most frequently, they're random cathinones - hardly a stand-out class of drugs - anyones guess which ones), especially if you're in the UK (post cathinone ban, there aren't many attractive drugs to use).