It is made by the twining of pairs of threads - wound onto "bobbins". The partially-made lace is held in place by fine pins that hold the intersections between two pairs of threads. When the lace is finished, the pins are removed and the lace holds the correct shape. Sometimes the lace was then starched (for example collars or cuffs) to make it rigid.
In period, the main use for bobbin lace was for collars (the traditional Elizabethan collar), turned-back sleeve edgings and as edgings along the dress (eg passements along the stomacher or metallic lace along the seams).
One of the best sources to see period bobbin lace is, of course, "Queen Elizabeth's wardrobe Unlock'd" - where there are many pieces of lace (Elizabeth was aparrently quite fond of lace). However, it is often difficult to tell whether the lace is bobbin or needle lace.
While needle lace was preferred, it was *very* expensive due to the vast amount of time it took. As far as I've been told, bobbin lace was often employed as an alternative and, where possible, copied the style of the needle lace. To give a basic understanding of why, bobbin lace generally takes between 1 and 2 hours to do a square inch of lace. Needle lace takes longer...
While quite versatile, bobbin lace suffers from inherent limitations of material that needle lace does not. Consequently, you can quite often identify which is which by looking carefully for such things as triangles in the pattern (a sure sign of needle lace).
Most modern bobbin laces are definitely out of period - this includes Torchon, Honiton and Bedfordshire (beds) laces. The main type of bobbin lace that was in period was called passements which looked like elaborate arches filled with filigree. These were generally used in a repeating pattern (often two shapes interleaved) and were used at first to edge collars - both for males and females.
Early on, the passements went around a square or circular collar. Later, the passements were often attached to a wider pattern consisting of squares filled with filigree, and this was generally made of needle-lace, though bobbin-lace was also used in these (especially where the person couldn't afford the expense of the needle lace).
The Elizabethan collars were of the same basic pattern, but made in a circular fashion. They were made over a circle in size so that they could be ruffled and the final product would be a circle. The collars were often made with multiple layers sitting on top of one another to increase the "ruffliness" of the lace, these layers starched into wave-patterns.
Metallic laces and other edgings were also frequently made from bobbin lace and often went down the front seams to the full length of the dress.
Have a look through Unlock'd and see the different styles...
If you want to learn to make bobbin lace, I'd recommend learning the modern styles (eg Torchon) which are much easier - then advancing to passements afterwards. There are period pattern-books around- if you find a copy, tell me and I will buy it!
Please Note: "grounds" of any sort are definitely Out Of Period, e.g. Torchon grounds etc - all period bobbin lace was made of filigree-style brides and interlinking sections, though there were sections made of cloth-stitch to make shapes. To get an idea of the "feel" of the type of bobbin lace, look at some Cluny lace and see how it is generally built of loops over-laying loops.
Here are some articles with instructions on how to make bobbin lace: http://lochac.sca.org/lochac/artsci/articles/lace.html http://www.florilegium.org/?http%3A//www.florilegium.org/files/TEXTILES/idxtextiles.html
Here is a website with instructions on making period bobbins: http://ilaria.veltri.tripod.com/bobbins.html