Hilton hotels tend to be larger, more full service properties with better amenities and a higher level of luxury.

Hilton Garden Inns caters more to a mid-level corporate business traveler. The rooms are comfortable, and generally spacious, but the hotel often does not have a full restaurant serving 3 meals a day, it often does not have a bar or lounge in it, and it generally won’t have a full service-oriented staff (no concierge, generally no bellmen, often no room service, etc.)

Hope it helps.
M

Hiltons are full-service hotels with a restaurant, bar, room service, bell staff, and concierge.

Hilton Garden Inns are limited-service hotels which are geared to business travellers. So they do have a small restaurant that usually serves at least breakfast. They usually also have a business center.