Z Canis Majoris is a young stellar object, one of the "Orion variables". The Orion variable classification got its name because the Orion region is so full of active and recent star formation that many of the young, eruptive variables are found there. Since these stars are eruptive, they're usually classified as Irregular or "I", and since many are also found in regions with nebulae, they also have an "N" for nebular attached. There are a number of subclasses of stars in this category, including perhaps the most famous, "INT" -- T Tauri stars. The specific classification given to Z CMa is "INA", which in plain text is simply "irregular, nebular variable, with early spectral type" (Z CMa has a spectral type of B8 -- slightly hotter than Vega which is an A0).