I am getting started with PICs and am ordering a PICKIT3 and some pic12f675s.

On Newark, I see several variants of the PIC12f675 listed.

For example:
PIC12F675-E/P
PIC12F675-E/SN
PIC12F675-I/P

Are there any major differences?

 

Yup, I/P and E/P are 8 pin DIP packaging, E/SN is surface mount SOIC-8 packaging.

The difference between I and E is smaller, the E version is rated over a larger temperature range, not usually worth worrying about.

Full details are right at the end of the datasheet.

 

Visit your fine manual and look at the section titled "PRODUCT IDENTIFICATION SYSTEM."