What is a weed, How do weeds harm gardens and Why are weeds hard to control?

What is a weed?
A weed may be broadly defined as any unwanted plant,
or as I have heard it said many times, “a weed is a plant out
of place.” For example, a corn plant is a weed if it is in a
strawberry planting. Most weeds are plants that are generally
considered undesirable by gardeners. They are neither
eaten nor considered attractive. They are frequently invasive
and difficult to control.
How do weeds harm gardens?
Weeds compete with crop plants for moisture, nutrients
and light. They may also harbor insects that harm flowers
or vegetables or transmit diseases to the crop. Weeds can
also serve as alternate hosts for diseases. They also promote
diseases by increasing humidity, decrease vegetable
quality and make harvests difficult. Vegetable plants grown
under weedy conditions will have reduced yields, or they
may not survive to produce at all. A weedy garden or flowerbed
is also unattractive.
Why are weeds hard to control?
Weeds are highly adapted to where they grow. They
have large, efficient root systems, grow rapidly and frequently
produce tremendous amounts of seed. Most weeds
tolerate drought and low fertility. Little of their energy is
used producing lush foliage, large seed or fruit. Therefore,
they often thrive where vegetables struggle to produce.
Many spread rapidly by vegetative structures as well as by
seed. The principle methods of weed control fall into three
categories: (1) mechanical, (2) cultural and (3) chemical.