It’s not safe to operate a tube guitar amplifier with no speaker connected. The amplifier needs to see a “load” — normally from a speaker — that gives the power it puts out somewhere to go. Without a load, a tube amplifier will likely be damaged.
If you want to use your amp silently, you can put a “dummy load” on its speaker outputs, which will give the wattage somewhere to go without turning it into sound through a speaker. Which brings us to your questions; there are two basic kinds of dummy loads: resistive and reactive.
The most common kind of dummy load is a resistive load, as is found in load boxes such as the Mesa Boogie Cabclone, which also offers a speaker-emulation circuit allowing for silent recording or use on silent stages. These load boxes consist of resistors that soak up the wattage being output from the amplifier and dissipate it as heat. Resistive loads are simple, which tends to reduce their price.
The other type of dummy load is a reactive load, as is found in products such as the Two-Notes Engineering Torpedo Live or Rivera Rockcrusher. These load boxes contain a network of resistors and capacitors designed to emulate the impedance curve of a speaker. A reactive load is generally more expensive than a passive resistor based load, but some players feel that it provides results closer an amp pushing a speaker.
Either type can be safe for your amp and can provide great results. Which one is best for you? The answer depends on your preferences, your budget, the sound you want to hear, the amp you’re using, your application for the load box, and more.