Small packs, such as waist packs or fanny packs (this term always conjures up memories of Albert Brooks and Michael Douglas in "The In-laws"), are not strictly speaking backpacks, since they are not worn on the back. Nevertheless, they serve the same purpose on a smaller scale and can be ideal for short hikes.

These smaller packs normally have a strap and two or three compartments and can be used to hold a number of handy items such as a water bottle, nutrition bars, sunscreen, insect repellant, band-aids and other small items.

Some smaller packs, often called hydration packs, even have special water bladders with drinking tubes. These normally hold anything up to a couple of gallons and again can be useful. You should remember though that a gallon of water weighs about 8 lbs.

If you're going on longer hikes, or need to carry more with you, then there are lots of different backpacks to choose from. Most of these are categorized by size, which is often expressed in terms of their volume in liters.

Day packs, as their name suggests, are designed to be used on reasonably short hikes. They are normally anywhere from about ten to thirty liters in volume and come in various styles. They will all have shoulder straps and some will have a chest strap and/or a belt strap.

Larger packs, from about thirty to seventy liters, go by a variety of names including, midrange, midsize, light duty packs and others. Packed correctly, they can hold quite a lot of equipment and it's easy to get carried away and try to pack everything including the kitchen sink. Be careful and remember that, whatever you pack, you have to carry it.

These backpacks need to be of a sturdy but lightweight construction and should have good broad shoulder straps and a waist belt. It's also very helpful if they have the now commonly seen plastic 'dog-leash' quick release buckles.

The largest packs, which have a capacity in excess of seventy liters and usually have a number of special features, are generally referred to as heavy duty, full-sized, expedition or something similar.

These large backpacks normally have special supports threaded through the shoulder straps and across the back or waist to help stabilize the backpack and make it easier to carry. They are also often designed to be carried higher on the back to keep the load off the lower back and so help to prevent back pain and fatigue.

The best full sized packs are manufactured from extremely strong composite materials, which are just about indestructible, and come with all sorts of lumbar support, pads and well designed balance mechanisms. Many will also have special compartments to hold all sorts of different things including sleeping bags or even a small tent.

At the end of the day the most important thing to remember when it comes to choosing a hiking backpack is that you will have to carry it. So, think very carefully about just what you need (not want) to take with you before you set off and pack this and nothing else.