Intel Celeron family is a line of budget x86 processors based on Pentium designs. Originally based on Intel Pentium II architecture, the Celeron processors migrated over time to Pentium III, NetBurst (Pentium 4) and Core architectures. Priced lower than their Pentium counterparts, the Celeron processors have certain high-end processor features disabled. For example, P6-based Celerons had multiprocessing disabled, while more modern CPUs may have disabled Hyper-Threading, Virtualization, AES instructions, and/or other features.
The Celerons are slower than similar-clocked Pentiums due to smaller size of L2 cache, and possibly slower bus speed. Celeron CPUs are usually packaged the same way as Pentium or Core-branded processors, and can be used in motherboards designed for Pentium/Core microprocessors.
For more information about the differences between Celeron and Pentium CPUs please see Celeron vs Pentium page.