Linux Man-pages and System Calls

Official docs outline

Section 2 syscalls list
Complete table of Linux system calls.
open(2)
Opening files, flags and file descriptors.
fork(2)
Process creation by duplicating the caller, copy-on-write and the return value convention.
execve(2)
Program execution and argv handling.
mmap(2)
Memory mappings and protections.
epoll(7)
epoll_create, epoll_ctl and epoll_wait with level-triggered and edge-triggered modes.
signal(7)
Signal semantics and async-safety.
futex(2)
FUTEX_WAIT and FUTEX_WAKE primitives underlying userspace locks.
ptrace(2)
Process tracing used by debuggers.
capabilities(7)
Permitted, effective, inheritable and bounding capability sets and file capabilities.
socket(7)
Socket options and generic behavior.
pipe(7)
Anonymous pipes, FIFOs, PIPE_BUF atomicity and blocking behavior.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.