Raised bed gardening takes on a few definitions, however they are all actually the same thing. A garden bed that is raised up from ground level.
Some raised bed gardens are nothing more than a pile of soil on top of the ground. A little more involved can be accomplished using railroad ties to make a rectangular or square and fill that area with soil. Really any kind of wood can be used, 2x lumber such as 2×4, 2×12 etc. are excellent choices.
Another idea is to build a more permanent structure, maybe of cinder blocks or rocks, raising the level up as much as waist high. This really allows for easier gardening because you don’t have to bend over to reach the ground. You may not be able to use your usual gardening tools like the shovel, hoe etc., because the handles will not be too long.
Any way you do it, from piling the soil up, to making some type of surround to contain the mounded soil, raised beds are a very nice way to garden.
It is worth noting that pressure treated lumber, real railroad ties as well as other lumber you may salvage or want to recycle is, or may have been, treated with chemical to preserve it and make it last longer. Choose your wood wisely if you are recycling products. Remember also that flower beds are different than vegetable gardens. Be sure not to use pressure treated, or otherwise treated wood, where you will be eating food grown in contact with these woods.