What is the difference between French and Swiss lace?

French lace can be two different things.

In the context of a non-hand-tied wig, French lace is generally lace which has a floral pattern on it.  You’ll find this mostly on standard / traditional cap wigs. 

French lace can also be a term to describe a rougher and more durable type of monofilament lace with a slightly larger grid pattern on it than Swiss lace. 

Swiss (monofilament) lace is a very fine translucent mesh.  This is the one you’re most likely to find in wigs with high-end features. (See below for detail.)