what is libc?

[iam at sdf.org]

i didn't know what to search for, hence directed this to the list.
what is libc exactly? is it the only interface between userland
and the kernel? or does libc do something more? if yes, may i know
what other functions does libc perform?
also, if suppose the userland were to be written in say "rust",
would it theoretically be possible to write libc in "rust", and
then would it have to be called "librust"?
if libc isn't only about "c", why isn't it called libsys? would
appreciate pointers to material to read to understand the above
in more depth than is possible via email.
thanks.