datasheets are your friends
Datasheet for each microcontroller will show you what packages are available and how each of them look like (yes that "extension" is the package type) ..
look for example 16F628A datasheet:
https://www.electro-tech-online.com/custompdfs/2009/02/40044F.pdf
page 163: PACKAGING INFORMATION
18-Lead PDIP: extension is -I/P
18-Lead SOIC, extension is -E/SO
20-Lead SSOP, extension is -I/SS
28-Lead QFN, extension is -I/ML
the extensions actually tell you two things .. the part before "/" is "working conditions" .. so I stands for "industrial" E for "Extended" or something similar, not sure ... the second part (after /) is the package type ...
If you are just starting ...try to get /P as those are trough hole DIP/SIP packages
if you go further down the datasheet, pages 164-167 you see pictures and measurements of all packages the PIC in question can be found ...
btw, if you are just starting, go for 18F chips as the price is same and they are "more powerful" and easier to work with... you also have free C18 c compiler from microchip (and it is much faster / easier to program uC in C then in assembler)
welcome to the wonderful world of microcontrollers