mysqld is the MySQL server. It provides database access to client programs, so it must be running or clients cannot use databases administered by the server. When mysqld starts up, it opens network ports to listen on and then waits for client connections. mysqld is multi-threaded and processes each client connection using a separate thread to provide concurrency among clients. Queries that write to the database are executed atomically; when the server begins executing such a query, it will execute no other query for the table involved until the current query has finished. For this reason, no two clients can ever modify the same row in a table at the same time.
Standard Options Supported by mysqld
--debug --port ...