A cutter lets you open a cigar cleanly instead of biting it. The cap keeps the wrapper from unrolling. If you cut below the cap, the wrapper can come loose; if you do not cut at all, there is no draw. Most people use a guillotine (straight cut), a v cut, or a punch.
A sharp blade matters more than the brand. A dull blade crushes the head and squeezes the bunch. Keep the cutter clean, and do not twist it through the cap.
Smoke Rings sells cutters in its shop. The encyclopedia does not tell you which cigar to buy. See cutters for what is available, and cutter types for more about guillotine, V-cut, and punch cutters.