Aspic is a dish in which ingredients are set into a gelatin made from a meat stock or consommé.
Head cheese is not a cheese but a terrine or meat jelly made with flesh from the head of a calf or pig (sometimes a sheep or cow), and often set in aspic.
While visiting Transylvania I spent several days with Hungarians who served me head cheese (disznófősajt) with breakfast, along with other traditional foods.
Gelatin and meat. Perfection.
It was amazing and inspired me to start making my own. First version was simple. Left over stock with meat (not from the head) thrown in and left to cool. Next up maybe something from my homeland like Cajun Hog Head Cheese. Now to freak my flat mates out when I bring home a hog's head.
Disclaimer: Not awarding answer for links to random recipes unless they answer some of the additional questions above ;)
So that's the English for Polish "salceson"! It was quite popular when I was a kid, very often the cheap, the only available meat product. I don't remember it that fondly, as often it was simply made with whatever pig leftovers they had at the factory... so you had to watch out for pieces of cartilage or... pieces of hairy ears ;-)
But then you could also find a good quality, especially made of tongues... so good!
It's also quite popular in Poland to make pig feet "jelly", simply cook some kind of bony and full of cartilage meat (fish work as well) with favorite spices, herbs, veggies. Then clear the liquid well (through cheese cloth or similar). In a small bowl pour a bit of the liquid on the bottom and wait a bit till it settles. then put whatever you want on - slices of boiled egg, cooked veggies, pieces of meat etc. and pour the rest of the liquid. Eat cooled, with vinegar pour over it (that's how we were eating it, best ever!).