what is actually supercritical fluid?
The name supercritical fluid is fancy but don't be confused by thinking that it this would a very odd looking state of material. Supercritical CO2 is just a state of CO2 at a certain T and P, so it looks very much like gaseous CO2 i.e., it is a transparent clear "fluid" whose density and viscosity is higher than a gas but lesser than that of the liquid. If you are able to see an interface in those transparent cells (e.g. here https://www.youtube.com/watch?v=GEr3NxsPTOA), it means that the liquid phase and the gas phases exist in the system. We are able to see the interface (boundary) when the refractive index is different from the container walls, and the surroundings. In short do not expect a new form of matter in supercritical state. It is just like thinking what does a plasma look like? It very much appears a like an ordinary flame which is extremely hot.

Response to the comment "is supercritical fluid a special types of a liquid ? if no, why then some SCFs used as a solvent ? what if a small pot of liquid heated inside a large pressure chamber to its critical point? would it expanding fill the whole chamber like a gas or stay in its container like a liquid ?"

I am afraid you are mixing several concepts here. The key question is, how does one distinguish a liquid phase and a gas phase? Supercritical fluid is not a liquid because its density and viscosity are not close to liquids, nor it has a definite boiling or melting point (a liquid does, right?). Think of supercritical fluid as a state or condition of a gas where it is above its critical temperature and pressure. That is all. We cannot say supercritical carbon dioxide has a boiling point of x Celsius or a freezing point of y Celsius. 

Coming to your second part on SFC extraction: First, let us talk about gases. Can the gases act as a solvent or carrier of molecules? The answer is yes, and this is the very basis of gas chromatography. How do you detect odors? The gas molecules are carrying the odor causing molecules from a far away place to your nose i.e., gases also have some capability of "dissolving" substances. In the same vein, if I use a temperature of Tc, and a pressure Pc for carbon dioxide (or any gas), we can use it for extraction. It just turns out that supercritical carbon dioxide has higher dissolving power than gaseous carbon dioxide and certainly less than liquid carbon dioxide.

Third point: Let us do an imaginary experiment: If you have a sealed container of fixed volume containing nothing but carbon dioxide at very low temperature. Also assume a pressure gauge is also attached. You have filled in enough carbon dioxide in the chamber that the gauge shows a value of 85 atm. Now you start heating the chamber until the temperature of the chamber reaches 35 Celsius. Imagine you can see the insides of that chamber with a camera: You will see nothing because there will be no liquid in that chamber. It will look empty to your eyes. Given the fact you know that an empty glass is not empty, it is filled with gases (nitrogen, oxygen etc). In the same way, our chamber is also filled with a carbon dioxide which our eyes cannot see. We will still call this carbon dioxide as supercritical carbon dioxide because we know from our experimental settings that we are above critical temperature and pressure of carbon dioxide. You can go higher temperature to 45 Celsius, still you are in supercritical state. Thus in a phase diagram, there is a supercritical region.

Don't let the word fluid confuse you. Since we cannot call a gas above its critical temperature and pressure a typical gas, nor we can call it a liquid, it is best to call it a fluid. Hope that clarifies your confusion.