Good shelter, be it a tent or bivi bag, is vital if you're planning on spending several days in the hills. Backpacking tents come in a range of weights, sizes and styles. The two most common modern designs are the geodesic tent and the tunnel tent. Geodesic (more round in appearance) tents are popular because of their large internal space, ease of pitching, strength, solid design and the fact that they are free-standing (ie hold their shape without pegs). A tunnel tent on the other hand, is much simpler in design, often lighter, and has excellent wind resistance when pitched into the wind.
Tents are given a season rating, like sleeping bags
Tents are given a season rating, a little like a sleeping bag, ranging from 1 for low level summer camping, to 5 for high altitude or expedition use. This is a very rough guide to the tent's strength, as one manufacturer's summer tent may outperform another's mountain tent due to the design, construction and quality of material. Tent specifications also show the number of people that can live in the tent, which may be anything from a single camper to a circus troupe, (although most manfucturers seem to think that campers do not not need a great deal of space!)
The usual colour choice is between olive green and garish yellow
The colour of the flysheet (the outside of the tent) is only an issue if you plan on wild camping. The usual choice is between olive green and garish yellow, depending on whether or not you wish other people to know you're there!
Sometimes you might want to travel without your tent, perhaps in order to minimise the weight or pack size you're carrying if you're walking far, or when the weather looks like it will remain good. In these situations a lightweight bivi bag could replace your tent. The drawback of bivi bags is that there is no shelter for cooking, and entering or exiting in bad weather without getting your sleeping bag wet is extremely difficult.
Rab hooped bivi bag
A bivi bag is nothing more than a waterproof (and sometimes breathable) cover for your sleeping bag. It also increases the insulation of your sleeping bag by about 5°C. Designs vary: from top expedition models using expensive breathable fabrics and featuring large over-the-head 'cowl' closures, down to simple budget models that have the same lay out but made from cheaper fabrics (which may not be breathable). Another more specialised design of bivi bag uses a small nylon or alloy hoop over the head of the bag to form a space around the face, thus creating a micro 'tunnel' tent.
One of the most popular bivi designs uses lightweight breathable fabrics, which are cut into a simple mummy shaped bag, devoid of zips or cowls, with reduces bulk and price, and is ideal for damp locations where you will not be directly exposed to the elements.
Do not confuse plastic, non-breathable 'survival' bags with bivi bags. These bags are designed purely for survival and not for comfort.