Great definitions, just adding a few details -
Record of Employment (ROE): Document issued by Service Canada for Employment Insurance (EI) purposes usually after the employment is finished. You can request one from your employer anytime you want.
You can also sometimes download this from the Service Canada website. Not required, but sometimes a useful document if the Letter of Employment/Reference letter is lacking.
Employment Reference Letter: A letter issued by your employer in a company letterhead stating your position, responsibility, term of employment, salary etc.
And for CIC's purpose, the letter of employment/reference letter attests to your completed or past work experience. It is required for all claimed work experience.
Offer Letter: An offer letter issued by the employer when the job offer is made. It contains all the terms and conditions of employment plus signature of both the parties.
Not required UNLESS you are claiming "arranged employment" (for 600 points). In addition to the LMIA, the job offer letter must indicate an offer of permanent full-time employment contingent upon the employee obtaining permanent resident status.