Style Small Group Tour Small groups are usually defined as between 10 and 24 travelers, often less. If you're the kind of person who enjoys more intimate experiences and personal service this is a good choice. All else being equal you will pay a premium for this style vs a larger group tour.
Private Guided Private tours give you the undivided attention of a guide, and often involve special access to sites and unique experiences not available to larger groups. This is a great option for families, couples, and small friend groups. Expect to pay a bit more for the extra service.
Self Guided Tour A travel company plans your itinerary and arranges all the logistics including lodging, local activities, and transportantion. You have the flexibility of a solo trip while still getting the convience and time savings of expert planning. Get 90% of the benefits of a tour, without a guide.
Group Tour Groups are between 25-60 people, typically ~30-40. Usually there will be many opportunities to split off and enjoy meals and excursions in smaller groups. This is the most economical way to travel, saving up to 40% versus booking the same itinerary yourself.
River Cruise These vessels are smaller than most ocean cruisers, limiting which amenties are available. Passenger counts can vary. One of the biggest advantages of a river cruise is the ability to dock at smaller ports and local villages.
Small Ship Cruise Small ship cruises usually have a max passenger count of 500. The primary purpose of these trips is to spend time off the vessel in local ports (e.g. Mediterranean) or experiencing nature (e.g Galapagos or Antarctica). Cabins can vary from budget to luxury.
Large Ship Cruise This is the "floating city" experience, with multiple ways to enjoy your vacation aboard the ship as much as on land. Ships are multiple floors, provide several activities, culinary, and shopping options. They often make fewer stops and have less time available for shore excursions.
Retreats Similar to a self guided tour but staying in one location only with activities and day outings included as a part of the stay.

Accommodation Camping Typically involves most nights sleeping in tents (sometimes permanent tented sites) or in rustic cabins and lodges.
Basic - 2 star You'll stay in no-frills, but clean and comfortable, hotels or guesthouses. A 'Basic' trip might also involve a few nights of camping.
Value - 3 star Mid-range budget with accommodations ranging from comfortable lodges, guesthouses, and homestays to three star hotels.
Premium - 4 star 3 to 4 star western hotel equivalents. While not all lodging will be 'luxury' they will be quite comfortable by western standards.
Luxury - 5 star The highest level of comfort and service. All accomodations are in four or five star hotels, boutique lodges or high-end homestays.

Physical Level Very Easy Minimal walking - motor vehicles available for all major parts of trip.
Easy Normal generally flat walking in urban or suburban environments.
Moderate Walking or physical activity half to most of day - no carrying equipment.
Strenuous All or most of day hiking or biking, hills included.
Extreme Very challenging all day hiking and backpacking carrying significant equipment.