User space applications often rely on Application Programming Interfaces (APIs) to perform their work. A library is essentially a collection or archive of APIs, allowing you to use a standardized, well-written, and well-tested interface (and leverage the usual benefits: not having to reinvent the wheel, portability, standardization, and so on). Linux systems have several libraries; even hundreds on enterprise-class systems is not uncommon. Of these, all usermode Linux applications (executables) are "auto-linked" into one important, always-used library: glibc – the GNU standard C library, as you shall learn. However, libraries are only ever available in user mode; the kernel does not have libraries (more on this in the following chapter).
Examples of library APIs are the well-known printf(3) (recall, from Chapter 1, Kernel Workspace Setup, the section of the man pages where this API can be found), scanf(3), strcmp(3), malloc...