Chocolate flying-saucer shaped things (although considerably smaller), covered in hard brown candy. If you're not careful, the shards of candy that result from biting into one can cut your mouth and gums to bits.
Minstrels are the sweet that, according to old advertising, would melt in your mouth and not in your hand. They're made by Nestle - the same people that make Galaxy, Cadbury's old arch-rival, and you can certainly taste the same kind of chocolate in these, Galaxy chocolate bars and other stuff made by Nestle, like Mars bars. The chocolate is smoother and heavier, and, frankly, not as nice.
In practice, Minstrels can actually melt anywhere they feel like it, be that mouth, glove-compartment or handbag. They become hugely sticky, and rather sickly when they've melted - they are considerably less fun to eat when you have to disentangle them from the bag. By the same token, mind you, if you leave them in the fridge, they are perfectly capable of cracking crowns and removing fillings, the nasty little swines.
The trick with Minstrels, and Maltesers come to that, is to leave them in your mouth until everything's gone as soft and soggy as it can. In the case of Maltesers much fun is to be had by drying and putting the little honeycomb ball back in the bag for unsuspecting other hands (and hopefully mouths) to find. Minstrels are just fun to eat when they're soggy. They're also fun to eat whilst seeing how many you can cram in your mouth; this though, works very much on the law of diminishing returns.