These electric heaters use metal coils running through them to heat the space there in. The coils are constructed in such a way that they heat very quickly and you will be able to feel heat come from them almost as soon as they are turned on. One problem with the heated coils though is they get red hot and can be a fire hazard if left unattended of something flammable comes in contact with them. If you purchase a coil based heater make sure that it has a feature that if it ever gets knocked over it will automatically turn off and be careful that when the heater is turned on that there is nothing that can potentially be blown into the heating coils and catch on fire and never them in a room unattended.
Ceramic heaters seem to be the next generation of coil heater and are a great choice as they do not use as much electricity as some of the other heaters on the market and pump out a fair amount of heat from a small unit and are also very highly portable. These ceramic heaters use ceramic plates to retain and distribute the heat by having their ceramic plates attached just a few coils at the edges so they heat up fast and the ceramic plate draws the heat from the coils to distribute the heat into the room. Ceramic heaters also cool down much faster than straight coil based heaters which make them a much safer electric heater to operate.
These are the heaters that look like old radiators and are filled with oil which is heated to produce heat for the room they are placed in and seem to use less electricity then the coil based heaters. These heaters are popular due to their portability, safety and being fairly inexpensive to run. They are also fairly safe to leave on for extended periods of time as there are no exposed heating elements that can catch anything on fire and due to their size work well at giving off consistent heat in order to maintain a lot of warmth in a room at a fairly low cost on your electrics bill.