If you are sewing a cross stitch or blackwork picture, then you need a floss. For a cross stitch you use either 2 or 3 strands of the thread, depending on which count you are using. For instance, if it is 14,16, or 18 count you would use 2 strands, if you are using 11 or larger it would be 3 strands.
Of course for blackwork you would use 2 strands for the outlines, or even cotton-a-broder, and 1 strand for the fill-in pattern, for tapestries you would use a woolen yarn.
Hope this is helpful to you, but if you are using a pattern, it should tell you which material to use, which floss to use, and how many threads to use, good luck anyway whatever you are doing.