A determinate mass of conidia-bearing hyphae that develops below the surface of the host plant and bursts through as the spores mature. It is distinguished from a stroma in not having a peridium or covering of fungal tissue of any kind.
A perithecium or pyrenocarp is a chamber within the ascocarp whose walls are lined with asci. The chambers are kettle-shaped, or shaped like a wide-bellied narrow-necked flask or laboratory beaker, with the mouth of the flask (or top of the kettle) opening onto the air and protruding a bit from the ascocarp. These chambers line the surface of the ascocarp so that the surface of the fruiting body is a fairly continuous series of protruding bumps, each with a hole (leading to a chamber) in the middle. This hole is known as an ostiole, and it is through the ostiole that the spores escape from the chamber.
There seems to have been some historical transformation in the characteristics of a perithecium; I'll have more on that when I finally put the Ascomycetes up. Perithecia are characteristic of the class Pyrenomycetes, and although " pyrenocarp" is obviously the term that has given the name to the group, perithecium is the preferred and normal term today.
A perethecium-like chamber that produces conidia instead of ascospores is called a pycnidium. I don't know of any research to find out whether fungi with both types of spore production, like the Xylariaceae, use the same chamber for both types of spores.
A locule is basically a perithecium in which the chamber develops first and then the perithecium grow inside it. With perithecia, the asci develop first and the chamber then develops around them. It's difficult for me to characterize locules per se, because they are found only in the Loculoascomycetes, and the asci in this class also have certain special features (they are bitunicate, for example, but not fissitunicate; the asci in perithecia are mostly unitunicate, with a few fissitunicate in the Lecanorales). It's hard for me to say whether these features are to be attributed to the definition of the taxon or to the definition of a locule. I suppose I'll have to take a stand on that eventually, when I add the Ascomycetes to the website.
A stroma is, on the one hand, a cushion-shaped mass of tissue that contains the spore-bearing structures ( perithecia or whatever). It is often formed just below the surface of the substrate (or host) and expands and breaks through the surface at maturity. We say that stroma which break through in this manner are erumpent. In this sense, the term is not restricted to Ascomycetes, as some Deuteromycetes, rusts and smuts also fruit in this manner. Here, for instance, we see the stroma of the former Sphaeria berberidis, the anamorph of the famous grain rust Puccinia graminis. It incorrectly shows the fungus as having asci, since in 1817 all fungi were thought to have asci.
The other usage of stroma is for any thick-fleshed (that is, not a Discomycete) ascocarp. The fruiting bodies of Xylaria polymorpha, for example, is not particularly cushion-shaped; but it is still called a stroma.