What is the Difference Between Silk & Satin?

Usually the answer to this question is something like 'Silk is natural & more expensive, satin is a cheaper man-made alternative.'
WRONG!
This must be one of my biggest sewing peeves. Obviously being a seamstress & milliner I have learned about fabrics, their weaves, their fibres & their uses, & I can excuse those who don't sew for not knowing. But the number of sewists & bloggers who claim to have a clue, get this answer wrong.
The difference between silk & satin is, silk is a fibre & satin is a weave.
SILK is the content of the fabric.
Like wool, cotton, cashmere, linen, viscose rayon, polyester, nylon & tencel etc, silk is a thread  & is derived from silk worms. If you look at any clothing tag it should list the fibre content that is either natural or man-made or a blend of fibres. You should not see satin listed here.

Silk in the raw looks like this.

 SATIN, however is not a fibre, but a weave that typically has a glossy surface and a dull back. The satin weave is characterised by four or more weft yarns floating over a warp yarn or vice versa, four warp yarns floating over a single weft yarn. 
For example

Source Study Blue Textiles Flash cards

So you can have silk satin which is more expensive & luxurious, it will fall differently to a man-made fibre content. Equally you can have poly, nylon or acetate stains, which are cheaper & don't have the same feel or sheen. Other natural fibres can also be used for satins, such as cotton, which is usually known as sateen, & even wool.
There are some fabulous man-made satins produced these days, & when it comes to something like the heavier weight Duchess or Elizabeth satins they can be be quite stunning & reasonable hard to distinguish, even to the trained eye/touch. I've been fooled on more than one occasion.
And when you want the dress of you dreams at a more affordable price, they are a good alternative.

Here endeth the lesson on silk & satin.