We have 4 normal types which are almost identical:
ES-030

RS2

Operating voltage: 4.8 < 6.0 v
Positioning time: 4.8 v I 6.0 v: 0.19s/60° I 0.11s/60°
Operating torque 4.8V I 6.0V: 3.2 kg.cm I 3.5 kg.cm
weight: ca. 39g
Torque: 3kg/cm
Gear: plastic

Parallax standard servo
rc car servo_4519

HobbyKing™ HK15288A Analog BB/MG Servo

Torque: 9kg @ 4.8v, 8kg @ 6v
Weight: 51g
Speed: 0.21 / 60deg @ 4.8v, 0.20 / 60 deg @ 6v
Voltage: 4.8v~6v
Plug: JR style
Metal Gears
Dual Ball-race

 Brown = 0
 Red = 5V
 Yellow = Control

We also have a few adapted servo's who now are continuous rotating servo's. They do not rotate to a certain position but
rotate continuously with a controllable speed and direction.

An explanation on how servo's work can be found here.

Here you can find how to control a servo with your Arduino

If you want to design a wheel or arm that should be fixed on those servos, you can use this for the hole for the axis.