Julia is currently a single-threaded language (although it does perform asynchronous I/O). This means that the Julia code that you write will run sequentially on a single core of the machine. There are a few significant exceptions; Julia has asynchronous I/O that can offload network or file access to a separate operating system thread, and some libraries embedded within Julia, such as OpenBLAS, spawn and manage multiple threads for their computations. Notwithstanding these exceptions, most user-written Julia code is limited to a single core.
Julia, however, contains an easy-to-use multiprocessor mechanism. You can start multiple Julia processes either on a single host or across a network, and you can control, communicate, and execute programs across the entire cluster.