One type of moving company is referred to as "you-pack" moving. With this type of service, the moving company drops a trailer off at your house and gives you a few days to pack all of your belongings in it. The trailer may carry other moving loads or merchandise as well, and your things will be separated from the other things by a bulkhead.
To estimate how much space you will need in the trailer, the company will have you take an inventory of your things and put a space estimate together. After you pack everything in, you will measure how much space you actually used and you will be charged accordingly. A truck will pick up the trailer and deliver it to your new home or destination according to the schedule predetermined by you.
Another type of DIY moving company is one where you truly do it all yourself. This involves renting a van or truck, packing all of your belongings in it yourself and driving the van or truck to your new location. This option can save you the most money, but only if your move is short distance. For longer distance moves, you'd have to factor in the cost of gas, which would be shared with other customers in the case of you-pack moving.
To decide which type of move to make, consider the distance involved. If the distance is short, you can save the most money by hauling your load yourself. If the distance is greater, you may have other considerations, such as how your vehicle will get to the new location among other things.
The best thing to do is to list out all the factors before settling on one or the other. Consider how much the convenience of not having to drive a moving truck cross-country is worth to you. Also, check out some of the online calculators that DIY moving companies have available to help you calculate your cost.