I'm assuming you're talking about software. Perhaps it would help us more if you specified the software you're using.
I have no experience using software to alter the sound of your guitar, but can tell you about the hardware equivalents and what they do.
Preamps are the first stage of amplification. They usually include some sort of gain to allow you to overdrive the signal and produce a distorted sound that is used a lot in rock and blues. As a simplification, they are used to add overdrive to your sound, each preamp having a different "colour" or personality.
Amplifiers are the second stae of amplification, and generally increase the volume of the signal without overdriving it too much (this is a generalisation... some amplifier stages can also distort your sound). They generally add volume without colouring too much.
A cabinet is the box where the speaker is hosted. It can give a certain "colour" to your sound, so it's part of the overall tone of your guitar.
Pedals are compact units that allow you to introduce some effect to your signal chain. You can add a reverb, a delay (also called "echo" sometimes), phaser, flanger, etc. Some of these can be used before they reach the preamp, some between the preamp and the amp (in the so-called "effects loop").
I don't know what an impulse loader is.
If you want to emulate the sound of plugging your guitar to a combo amp, all you need is
Guitar -> preamp -> amp -> cabinet
If you want to introduce some effects, your signal chain could look like this:
Guitar -> effects that affect pitch and tone (wah-wah, overdrive, distortion) -> preamp -> time and phase effects (reverb, delay, chorus...) -> amp -> cabinet
There is not single "right" way to use them. The above is just a very common pattern. It's up to you to experiment to find your own sound.