‘A weed is any plant growing in the wrong place’ – but some weeds are more wrong than others.
Unfortunately weeds are not weedy but more thuggee.

Weeds: The Story of Outlaw Plants: A Cultural History

Problems Caused by Weeds
At their worst weeds can take over and suppress all other growth.
Weeds take water and nutrition from the soil and rob your plants.
Other cultivated plants can’t fight weeds and give of their best at the same time.
Weeds can throttle and kill weaker species and cast a shadow over others.

Pest & Weed Expert:

Declare War
Before we get onto chemical warfare you can fight weeds at there own game.
Cover the weeds with a membrane to exclude light. Cardboard or layered newspaper may work and you can plant through this covering. Add at least 2″ of mulch and soil on top.
Hand weeding is the Rolls Royce method. You have to keep up to this job on a regular basis.
When digging over a patch take care to remove all perennial weeds and bits of their roots. (Don’t compost the roots).
Annual weeds can be hoed so the roots can’t access water and thus die.

The Book of Weeds

Chemical Warfare
Weed killers are available from most garden centres.
Glyphosphate is a systemic chemical that enters through leaves to kill roots. It is ineffective and dissipated as soon as it hits the soil. Spray in autumn and spring on growing weeds but avoid your cultivated plants.
Selective chemicals are available for lawns.

Weeds and What They Tell Us

Gardeners Special Tips
Do not let weeds seed. Cut off weed flowers even if you can’t remove them.
Do not compost seeding weeds as they may not be killed by the heat of the compost.
Kill weeds when they are young.
Early recognition and control of weeds leads to savings in the cost of herbicides and aggravation

Weed Seedlings Colour Atlas

Other Resources

Most Common Weeds
Common Lawn Weeds
Uncommon Weeds
Weed control of Avens
Horsetail and Mares-tail

The Weeders Digest (of edible weeds?)