Mathematica allows arbitrary hierachical namespaces. However, several standard "contexts" are conventionally used for predefined purposes.
System` — the context for system-defined symbols
Global` — the default context for user-defined symbols
package` — the conventional context for public symbols in a package
package`Private` — the conventional context for private symbols in a package
$CellContext` — placeholder for the context of a symbol inside
Dynamic
Cell$n`,
Notebook$n` — unique contexts generated for cells or notebooks
Developer` — built-in symbols intended for advanced package development
Experimental` — symbols introduced in a particular version for experimental purposes
