"Sky Lake" Processors

Pentium/Celeron is what Intel calls 2 Skylake cores with hyperthreading and AVX instructions disabled.

i3 is 2 Skylake cores with hypertheading and AVX instructions.

i5 is 4 Skylake cores with hyperthreading disabled, but has AVX instructions.

i7 is 4 (or more) Skylake cores with hypertheading and AVX.

Every year Intel releases a new core design, with a new name. Skylake is the newest design. Last year's was Broadwell, then Haswell before that. Kaby Lake is next year's design.
 


I should have said, "With hyperthreading and AVX both disabled."