Single One cylinder. Eg: BMW F650
+ Few moving parts, good mpg.
- Low power, vibey.

Parallel twin Two cylinders side by side. Eg: Honda CB500
+ Ease of manufacture.
- Vibey engine.

Flat twin Two cylinders, flat, facing apart. Eg: BMW Boxer
+ Ease of maintenance, low center of gravity.
- Cylinders stick out, reducing ground clearance.

V-Twin Two cylinders in a 'V' shape. Eg: Ducati 999
+ Very narrow, good traction, sound great.
- For equal cc, can't make the power of a four-cylinder.

Triple Three cylinders, in a line. Eg: Triumph: Speed Triple
+ Character of a twin, power of a four (nearly).
- Power of a twin, character of a four (nearly).

In-line four Four cylinders in a line. Eg: Yamaha R1
+ Lots of cylinders mean plenty of power.
- Very few. Some complain of a lack of character.

V-four Like two V-twins side by side. Eg: Honda VFR
+ Compact and powerful.
- Complicated and expensive to build and maintain.

V-five Three cylinders point one way and two another.
+ Just look at the Honda RC211V.
- They haven't made one for the road. Yet.

In-line six Six cylinders in-line. Eg: Honda CBX1000.
+ Power, smoothness, noise.
- Too heavy and complex.

Also see The following oddities have all been used on bikes at some point: tandam twin (KR250), V-six (Laverda endurane racer), V-eight (Morbidelli), and rotary engines (Norton). Honda's NR750 was a V-four with oval pistons.