What is the difference in use between Form I-131 and Form I-191?

[kishoresojitra]

I am US permanent resident trying to get temporary work permit in Canada. One employer in Alberta is ready to give me sponsorship letter. My questions are: -
1) How much time it takes to get LMO by employer from HRSDC in Canada?
2) How much time it takes for USCIS to issue reentry permit document once we file I-131/I-191?
3) Which form is needed to file? Is it I-131 or I-191? Both forms seems for reentry permit for US permanent resident.
4) How much time it takes for Canadian consulate to issue TWP?

Thanks in advance.

Kishor.