Cloaks come in several varieties. They keep the rain off, and in period have detached hoods more often than not. Cloaks provide some protection form the cold, however an extra undertunic (thermals suffice until you can make enough) is often warmer, as cloaks blow open. Some tabards and surcoats (especially fur lined ones) are also good garments to wear for warmth. Cloaks are, however wonders in the rain, as a good wool cloak, even without waterproofing, can take a long time to soak through, and can be easily discarded when you go indoors.
Cloaks can also be used as picnic blankets, bedrolls and full circle cloaks can be large enough to shelter a friend too.
cloaks generally take a lot of fabric. Although other fabrics work, wool is a great fabric, and a cheap source of large lengths of pure wool is salvation army op-shops where blankets sell for $10-$15, often in matched pairs for the larger cloak. (cheap compared to buying 6m of fabric at about $6/m) Remember that many cloaks were worn to keep the wearer warm and dry. Furs are warmest as a lining, rather than on the outside (although in some cases fur on the outside might be symbolic), velvet wasn't invented till middle period, and was an expensive fabric mainly used for court garments, and shinny crushed velveteen synthetic fabric panne will mark you as a newcomer (but if you are, you'll be nice and warm still).
Cloaks range from very simple to make to moderately hard. Some early period "rectangular cloaks" are basically draped blankets and can be made with no sewing. Also poncho-like cloaks, requiring little sewing, were worn, chiefly by women at this time. As time progresses, full circle and half circle cloaks developed. Late in period some fitted cloaks developed.
see also: hoods