Ok. This kinda long but its really simple. When you truck you will most likely lease on with a company. You will sign a lease agreement and be a contractor for them. They will solicit customers and bid on a shipping price. Say they settle on $1000 for a load. You will haul the load and most companies take 22% off the top and you are paid the remaining 78% minus your fuel. This is customer freight.

Say another company lands the bid for $995 but cant cover the load. They will broker the load to usually the first qualified driver to agree to take the load. They do this by posting it on a load board. You or a dispatcher will see the load and call the broker. If all parties agree the broker will take a cut off the top and usually a big cut. Say $300 on this load. That leaves you with $695 minus 22%, minus your fuel. You can see where customer freight is WAY better than broker loads. As a rule of thumb, i am clearing nearly twice as much with customer freight vs brokers.
Also customer freight is usuall much better loads to haul as your company has hand picked the type of freight vs taking what other cant or will not cover.

To answer the weight question. It all depends on your vehicle. Mine, for example, is a straight truck. It weighs about 14k lbs. that leaves me with about 12k lbs to make up my gross weight. I don't put 12 on my truck though. I won't go over 10 b/c it put me over on my steers. The weight distribution will not let me, therefore I'm at 24ft and 10k lbs.

Different setups will allow for more or less weight and length depending on vehicle design, but all must be less than 26k total.