「What’s the difference between glibc and libc6?」
libc6 and glibc are the same version of libc; officially, it’s version 2 of the GNU C Library (but it’s the sixth major version of the Linux C library). You can read more about glibc at the GNU C Library pages.
libc6: Version 6 of the Linux C Library is version 2 of the GNU C Library; the confusion is because Linux has had multiple C library versions. This is the newest technology available, and includes features (like “weak symbols”) that theoretically allow new functions and modified structures in the library without breaking existing code that uses version 6, and avoid kernel version dependency problems. You should be coding and compiling all code against this version.