There are two main types of biasing: fixed biasing and cathode biasing. Fixed biasing does not mean the bias is not adjustable, in fact, it usually means the opposite. Cathode biasing is usually fixed, and not adjustable, and fixed biasing is usually adjustable with a small trimmer potentiometer, or "trimpot". It is no wonder the subject is confusing to people!
Fixed biasing means the tube is biased by means of a DC voltage, which is usually a negative voltage applied to the grid of the tube with respect to the cathode. As the negative grid voltage is adjusted, the bias current will increase or decrease, depending upon the direction the bias voltage is going. In general, as the bias voltage becomes more negative, the bias current becomes smaller, and the tube is biased "colder". As the bias voltage is adjusted less negative, towards zero volts DC, the bias current becomes larger, and the tube is biased "hotter". This is because a tube is a "normally on" device; that is, it allows current to flow from the cathode to the plate when the grid is at zero volts with respect to the cathode. The tube can be turned off, and the current flow stopped, by making the grid voltage negative with respect to the cathode. The tube can also be biased by referencing the grid to ground, or zero volts DC, and applying a positive DC voltage to the cathode. This is the same as keeping the cathode at ground and applying a negative DC voltage to the grid, because it is the grid voltage with respect to the cathode that determines the amount of bias current in the tube.
Since vacuum tubes are "normally on" devices, a trick can be used to bias them without having to supply a negative DC voltage source to the grid. If a resistor is placed between the cathode and ground, and the grid of the tube is referenced to ground (usually by connecting a large value resistor, such as a 1Meg, from grid to ground), the tube will try to conduct a large current from cathode to plate, since the grid and cathode are initially at ground potential. However, this cathode current flow will cause a voltage drop across the cathode resistor, making the cathode voltage positive with respect to the grid. Since the cathode voltage is now positive with respect to the grid, the current flow will decrease, and the tube will head back towards cutoff. A point of equilibrium will quickly be reached where the increase in current is offset exactly by the increase in cathode voltage, and the bias current will stabilize at some particular value. It will remain at this value unless the resistor value is changed, or a different tube with different characteristics is plugged in. This allows the desired bias point to be set by varying the value of the cathode resistor.
