Just like a regular individual RRSP, the money in a Group RRSP is tax-deferred and intended for retirement. The main advantages are:

Entirely voluntary enrolment
Employer matching is available, but not required
Payroll deduction of contributions means immediate tax advantages
Access to lower-than-retail investment fund fees
Very low administrative involvement for employers
Employer contributions to a Group RRSP are deductible as a salary expense to the employer, but are taxable as income on the employees T4.

Registered Pension Plans:

Mostly set up as Defined-Contribution plans today, Registered Pension plans are subject to more rules and restrictions than other registered groups savings products. Employer contributions are a deductible expense, and member contributions are tax deductible.

Employers are required to contribute a minimum amount, and rules typically dictate certain required contributions by members. Vesting is not permitted.