What you need to know when choosing a blade:
1) You need to know blade diameter & bore size (refer to your power tool manufacturer’s handbook/instruction manual. Once you’ve done this you are already halfway there.
2) You need to know the application required.
Very briefly a high number of teeth is ideal for cross cutting (including cross cutting at an angle or bevel angle on mitre &/ radial arm saw for example), typically from 48 to 120 teeth depending upon diameter of blade. High tooth count suits these materials: solid woods, plywood, laminated, veneered materials, plastics, non ferrous metals, chipboard, & M.D.F .
A lower number of teeth will suit rip cutting ( cutting along the length of timber), typically anything between 12 to 24 teeth & beyond if blade is large enough.
3) You need to know that: The gullet is the space between the teeth & allows the removal of cut material to be stored during the cutting process. A larger gullet is usually needed for ripping as the chips are more voluminous than when cross cutting. However a larger gullet does not yield a particularly nice finish; it’s not uncommon to see the tell tale signs of blade marks along the length of a piece of wood. A smaller or tighter gullet means a greater number of teeth or tooth count which permits a finer cut during cross cutting operations.
4) You need to know: Tooth Geometry; a blade is said to be Positively set if the teeth are leaning in the direction of rotation, This leaning angle is often referred as the Hook &/ Rake Angle. A ripping blade might very well have a Hook/ Rake Angle of say 15 degrees which makes for a fairly aggressive cut. Whilst a blade with no Hook/Rake Angle is said to be either ) degrees or neutral; typically metals, certain plastics. A blade with teeth leaning backwards in relation to direction of rotation is said to be Negative; normally minus 6 or 5 degrees so as not to tear through materials such laminated boards, non ferrous metals & plastics.