Inverter is an electrical device that converts DC (direct current) from the battery (12 volt automotive, marine etc. usually) to AC (120 volt, 60 Hz). The AC (alternating current) source available in the wall power socket in our houses is based on Sine wave. The inverters available in the market try to mimic that sine wave. The more closely an inverter produces the sine wave, the more circuitry/logic it needs which means more expensive it will be. I realized, not strictly speaking, with a same wattage rating specification, a square wave inverter will be half the cost of modified square wave inverter whereas the modified square wave inverter will be half the cost of PWM (Pulse Width Modulated) sine wave inverter. PWM sine wave inverter's output is the safest for desktop computers, computer servers and networking devices etc. provided it maintains PWM sine wave output voltage of 120 V AC (+/-5%). It's risky to connect use modified sine wave inverter with such high sensitive electronic devices. Here is an interesting picture for understanding these terms visually.
Figure 1: Plot of different waves. Source: http://www.reuk.co.uk/Pure-Sine-Wave-Power-Inverters.htm
Red color wave in figure 1, is a pure sine wave which is available at the wall socket. Square wave is in green color and modified square wave is in Blue color. Square wave inverter manufacturer will hardly mention it on the box but it will surely alert people not to connect it with electronic devices. Such inverters are mostly good for inductive loads (devices/equipments e.g. fan, motors etc.), modified square wave inverters are good for TV, laptops etc. as well but usually not for computers are servers. If your desktop/server's power supply (device that converts AC to DC) is from a good company and not the generic one, most likely it can take input from modified sine wave inverter as well but there is a risk factor here. Conclusion is, if you are going to buy an inverter for your furnace blower then save money by not buying PWM based inverter.