Several alternatives compete with UML, the list below gives an overview (just an arbitrary selection, not exhaustive):
- informal box- and line diagrams. They can be helpful and are often ok, but please explain your notation, otherwise your diagrams are open for (mis-)interpretation.
- Fundamental-Modeling-Notation (FMC), an (academic) approach to communicate dynamic systems with block-diagrams, petri-nets and entity-relationship diagrams. See the FMC notation reference for details. FMC is rarely used in business or industrial systems, although it’s quite promising.
- SysML is an UML dialect, created to support systems engineering. Supported by numerous modeling tools, but in my opinion not practically relevant.
- Simon Browns pragmatic C4 model of software architecture description.