Hotplates and stirrers are the laboratory equivalent of a stove-top. The hotplate surface is typically made from ceramic which allows them to be heated to temperatures over 500C and are resistant to chemical spills.
When choosing a hotplate, considerations are the surface area, whether the controls are digital or analog and the maximum achievable temperature.
For the most precise control of solution temperature, a thermometer should be used since the built-in thermostat only measures the temperature of the heating surface which can be hotter than the substance being heated.
Magnetic stirrers use a magnetic field inside the base of the unit to cause rotation on a stir bar that is inserted into the solution that needs to be mixed. These can be limited by the volume of the solution and the overall power of the field generated.
Overhead stirrers are becoming more common and deliver higher rpm and are capable of stirring larger volumes.
Hotplate-stirrers combine the two devices into one to achieve a well-blended, activated by-product.
Look for the maximum volume and temperature when comparison shopping.
Stellar Scientific carries hotplate and stirrers that are appropriate for chemistry, bio-chemistry and molecular biology based research.