A pre-sales engineer is the technical point of contact for the customer. That person needs to know the technical details of their speciality (Data Center, UC, Borderless, etc) and as much as possible about all other areas. Generally speaking they work with the customer, find out what problem they are trying to resolve, then work with them on product selection and design. Depending on the company (assuming Cisco Partner here), they may have additional roles such as working w/management on budgeting, product life cycles, training, etc. The level of involvement is on the person and how much trust they have earned from their client. AM's manage the relationship between the customer and Cisco/Cisco Partner company. They are not expected to have in-depth technical knowledge, but most have a good grasp on the basics (difference between and switch & a router).
Hope it helps.
