Roses or Ragweed; weeds can be defined as:
"Any foliage that you do not want where it's growing".
Weeding is the process of removing them.
Sounds simple enough, but there's more . . .

Weeds have two bad habits.
Above the ground, they grow fast to shade your chosen plants from sunshine. 
Under the ground, their roots rob your plants of nutrients. 
Some also tend to grow roots that intertwine your plant roots, 
and also wind around your plants above the ground.

There are three stages for stopping weed growth:

(1) - You can stop the seeds from germinating,
by eliminating seeds or destroying them.
(2) - You can destroy sprouts after they germinate.
(3) - You can fight with them all season.

(1) - It is best not to let weeds mature and produce seeds from the previous season. Seeds near the surface of the soil may be destroyed by covering your planting area with black plastic early in the Spring, and let the Sun cook them. Some commercial growers use steam or other substances to kill seeds in the soil.

(2) - You can plant a little later to allow weed seeds to germinate and then till the sprouts under. This method is most effective if you allow a second crop of weeds to sprout and till them under, too. It may take several years of this practice before you notice the improvement.

(3) - After weeds mature, you can pull them or Hoe them out.
The first step in weeding may be to water your plants, or wait for a rain. 
A couple hours after the water disappears, cultivate your plants with a hoe. 
Back away from your work so you don't leave footprints in the cultivated soil.

Hoeing will allow rain to penetrate the soil and encourage better root growth. 
It will also kill a lot of weeds, and loosen others. Then place walking boards in the paths so as not to make mud or compact the soil. 

Carpet remnants covering paths will retard weed growth. Carpeting or black plastic can also be used in the rows with small holes to plant your seedlings. This will also conserve moisture, and shelter Earthworms.

The trick is to remove the weed with its roots. Otherwise it will be back tomorrow. Some weeds have shallow roots that can be removed easily. Alfalfa may have roots fifteen feet long. There is no way you can pull an Alfalfa root, so the best alternative may be a "V"-shaped root cutter that will cut them off below ground level.

Most weed roots seem to be deep enough that they will break off just before they pull out. This is where the watering helps. Gloves will protect your hands, but they also prevent you from feeling the weeds. If you grab a weed at ground level and then give your hand a twisting motion as you press your thumb and fingers into the cultivated soil, you can gain an additional 1/2 inch or more of the weed stem at a point where it is tougher and less likely to break.

If a weed is growing tight against a plant, their roots may be intertwined. In which case use both hands; one to hold the plant roots down at soil level, while you pull steadily with the other. Often these will not pull out, so cut them off as short as you can. Usually you will not have this problem if you attack the weeds while they are still very small.

Don't remove the weeds from your garden unless they contain seeds.
They have removed nutrients from your soil, so removing weeds removes those nutrients. Instead, use them as mulch in the paths. Lay them with the roots facing away from you, parallel with the row as you weed backward. This will keep the roots on top of previous weeds and allow them to dry out instead of re-root.

These weeds will decompose as the worms and insects work on them from below. The soil will retain more moisture since it is protected from sun and wind as the nutrients go back into the soil. Diligence and determination are a Weeders' best allies.