Unified Modeling Language (UML) is a general-purpose modeling language intended to provide a standard way to visualize the design of systems. It is managed by an organization called the "Object Management Group" (OMG), and adopted as an ISO standard. It was developed with Software Engineering in mind, but has broadened out to be used in a variety of contexts, notably including Business Process Modeling (BPMN ), and Systems Modeling (SysML). The SysML specification (pre-v2.0) actually requires the UML specification as part of its “full spec”.
UML is currently in version 2.5, and contains over a dozen types of diagrams, split broadly into "Structural" diagrams and "Behavioral" diagrams. It is a large language when compared to IDEF (IDEF).
Specifications are available from http://omg.org/spec/UML. However v2.5.1 is 796 pages long. UML is big. But you only need to know a fraction of UML to make it useful.
UML is a tool. It can be useful, but it can also be entirely foregone in lieu of some basic informal boxes with lines.