You may not know Dennis Ritichie but you are using his work right now. He was instrumental in developing Unix (which is ultimately related to developing Linux, which you are using right now because this web site is delivered via a Linux server) but he’s more well known, probably, for his work developing the C language.
Every time an old computer language is mentioned for some historical reason, we learn that even thought we thought the language was dead, or obscure, or irrelevant, it turns out to be used in some wide range of functions and life itself would be impossible as we know it without it. I think with C it is the opposite. If you know anything about anything, you know that C or some variant of C is basic to everything. C is arguably one of the hardest languages to program in, but it is also the language that most stuff that needs to be efficient gets programmed in. It will probably turn out that it is not used anywhere for anything. (But I doubt that.)
(Code Trolls will tell you in the comments below the many ways in which what I just said is unconscionably wrong. I’m sure they’re right. I’m sure they’ll be polite, though.)
If you learned C you probably read K&R. R is Ritchie.