You can send signals to processes. Generally parents send signals to their children.
Pipes are used to redirect input and output from the shell to each other. This is done by creating two file data structures at the same VFS inode that point to a physical page in memory. One file can write, and the other can read.
The OS handles synchronization, by making sure that when a write is processed, the read also gets consumed from the file.
On writes, the process acquires the lock and gets to write to the page in memory. On reads, the process consumes bytes until it finishes, in which case it is put to sleep until signaled that there is more data (normally after the writer finishes writing).
Sockets allow for IPC as well. socketpair can be passed
AF_UNIX for IPC or AF_INET for Internet
protocol sockets, and then either SOCK_STREAM or
SOCK_DGRAM for TCP or UDP style communication, and an array
of ints to hold the socket fds.
Message queues allow one or more processes to write messages, which will be read by one or more reading processes.
Semaphores allow processes to signal between processes to operate on shared memory (like a file).