C++03 is an older revision of the C++ standard. The more recent revisions, C++11 and the current C++14, are still being adopted.
In 2003, the C++ Committee responded to multiple problems that were reported with c++98, and revised it accordingly. The changed language was dubbed C++03.
Though c++11 and more recently c++14 have superseded C++03, many compilers do not yet fully support the new standard. As a result, C++03 is still the default language standard used. Additionally, while the differences between C++98 and C++03 were primarily semantic, the C++11 standard added many new language-level features resulting in a greater impact to not only the compiler authors but to C++ programmers as well.
Please tag questions about C++03 with the c++ tag, along with the c++03 tag.