Here is my latest blog post on what options you have with your pension. I kept this fairly general as there are a number of moving parts to be honest and things that can be specific to your situation.
The first distinction between plans is that some are defined contribution plans, some are defined benefit plans, and some companies actually have an element of both. A defined benefit plan (sometimes called a DB plan) means that once you satisfy certain conditions of employment or membership of a pension plan, you are provided with a defined benefit. If you receive a statement from the pension plan that tells you that upon your retirement you will receive a pension of $X.XX per month based on your current status at a certain date, you are probably in a DB plan. With a defined benefit the investments are taken care of by the plan and they are promising to pay you a certain amount at a certain time. In other words as the name implies the benefits are defined.
Feel free to contact me if you have any questions or specific scenarios where you have questions.