Tar paper is used, among other things, for waterproofing roofs to prevent ingress of moisture. It is used as underlayment for asphalt, wood (a.k.a. shake), or other shingles, or even gravel, since tar paper itself isn't particularly wind- or sun-resistant.It is sold in rolls of various widths, lengths, and thicknesses (3 foot wide rolls, 50 or 100 feet long and "15 lb" and "30 lb" weights are common in the U.S.), often marked with chalk lines at certain intervals to aid in laying it out straight on roofs with the proper overlap (more overlap for flatter roofs).
It can be installed in several ways, such as: mechanical fasteners, or a combination of the afore mentioned. It is often applied with staples or roofing nails, but also sometimes applied in several layers with methods such as a torch, hot asphalt, cold asphalt (adhesive), non-asphaltic adhesives, and heat (torch, hot air) and additional hot tar.
Older construction sometimes used a lighter weight tar paper, stapled up with some overlap, as a water- and wind-proofing material, but modern construction uses 8 or 10 foot widths of "Housewrap," one brand of which is Tyvek, which is extremely durable and wind- and water-proof since there are far fewer seams than with the 3 foot wide rolls of tar paper.
Many new pitched roofs however use a TPO (thermoplastic polyolefin) membrane for increased protection against leaks. These membranes (which are usually made up of advanced fabrics) have advantages over traditional 1F roofing felt, in that they are more durable and less prone to puncture and tear. They are also lighter and stronger, though quite recent (2003) in the market. There are also breathable variations, which allow water vapour to pass through the felt; when used in conjunction with proper ventilation, they help minimize condensation in loft spaces.