`"] resets the current context.
Begin resets the value of $Context.
The interpretation of symbol names depends on context. Begin thus affects the parsing of input expressions.
See the Mathematica book: Section 2.6.10.
See also: BeginPackage, End, $ContextPath.
This is the current context when you start Mathematica.
This is the current context path. A symbol whose context name is not specified is looked up first in the current context, then in the contexts in $ContextPath.
This command changes the current context, but not the context path.
The following symbol does not exist either in the current context Temporary`, or in the contexts in $ContextPath. As a result, it is created in the current context.
This is its full name.
Following this command, the earlier context is restored.
Now newsymbol is not recognized, since the context it belongs to is not in the search path.
Information::notfound: Symbol newsymbol not found.
A very common use of Begin is to hide symbols defined in packages. This technique is illustrated in the Further Examples for BeginPackage.