Basically, there are four different weight classes of mechs which are:
- Light Mechs (20-35 tons)
- Medium Mechs (40 - 55 tons)
- Heavy Mechs (60 - 75 tons)
- Assault Mechs (80-100 tons)
There are many different mech chassis e.g. Hunchback, and you can get mechs in different variants to accomidate different weapons.
Light mechs are used mainly for scouting and sending information to your team- mates, and travel at very fast speeds, therefore they are fairly hard targets to hit, but have a little amount of armour and weapons.
Medium mechs can travel fairly fast but also pack quite a punch, and have a decent amount of armour.
Heavy mechs vary in speed quite a bit, some being much faster than others, and also pack a large amount of weapons and armour.
Assault mechs are the slowest mechs, but pack the largest amount of weapons, normally mounting a bit of everything, and also have a massive amount of armour, so whilst they are slow, they are very tough.
In mechwarrior online you will be able to fully customise your mech in the MechLab, which is where you do all of the work on your mech. In the mechlab you can change the Weapons mounted to your mech, the Engine, Armour and so on. There are however restrictions, these can be from the weapon being to heavy to be mounted to the mech or the Hardpoint restriction being reached (E.g. If you have a arm on a mech that can mount 1 laser in total, you cannot mount another laser in this arm). You can also add or subtract armour from the mech in the mechlab, which does effect the overall tonnage.
There are three main groups of weapons, which are the following:
- Ballistic- Weapons such as M-Guns, AutoCannons (ACs) and rifles, and require ammo
- Missile- Different types of missiles, the most common being LRMs (Long Range Missiles) and SRMs (Short Range Missiles), ammo required
- Energy- Weapons such as Lasers, Flamers, PPCs etc. Ammo is NOT required for these weapons, however they generate alot of heat- could attach more heatsinks to mech to help manage the heat generated.
Some mechs will not have hardpoints for all types of these weapons, so, for example a mech could only have ballistic and energy hardpoints, so no missiles could be equipped to it.
Note that a Light mech is able to hold weapons that even an Assault mech could hold, but just not as many, so bigger is not always better.
Hope this helped, tried to keep it quick and simple