Celeron is the marketing name used by Intel for its low-end processors.
This brand name was invented during Pentium II era, and is still used today.
But, if when they were first released, Celeron were low-end versions of Pentium II, it is not true anymore : for each architecture released since Pentium II, there have been Celeron versions.
So, there is not only one Celeron kind, but several, each based on different core architecture, with almost nothing in common :
Sandy Bridge Architecture