Chromosomes look like they have two sides, like an "X", with a connection somewhere, but not necessarily the middle. Each of the two sides is a chromatid. They don't always look like this, just when the cell is getting ready to divide.
Each chromosome (or chromotid) is made up of really long chains of DNA, that wind and loop back on themselves. It is the long strands of DNA that have the double helix shape. This is what you have seen drawings of--it looks like a ladder, twisted. Each "side" of the ladder is a series of phosphates and sugars. The "rungs" are base pairs. The drawings you have seen of the DNA double helix is a very, very small part of a chromosome; the DNA is millions of base pairs long, all wrapped and twisted into chromosomes.
Chromosomes are filamentous bodies which are typically present in the nucleus and which become visible during cell division. They are the carriers of the genes or units of heredity. The length of chromosomes varies from 1 micron to 30 microns. The chromosome includes the centromere, secondary constrictions, nucleolar organisers, telomeres and satellites.
Under the light microscope the metaphasic chromosome appears to be made up of two subunits called chromatids. These are possibly twisted around each other. If the chromosomes are treated with trypsin to remove proteins, each chromatid is seen to consist of two subchromatids. Possibly the subchromatids represent the chromonemata.
Right, the esiest way to understand this whole DNA thing is to work from the bottom up. There are bases called Thymine (T), Adenine (A), Guanine (G) and Cytosine (C). T pairs with A, G pairs with C all the time. These bases make up the double helix structure of DNA itself, which is supercoiled to make it really tiny.
Chromosomes are made of bundles of these supercoiled bits of DNA. Chromasomes look like little X's, in a chromosome there are 2 chromatids. The X that is a chromosome is made of two bits, the chromatids. If you can imagine splitting the X down the middle, each half is a Chromatid. It is during meiosis that the chromosomes are pulled apart into 2 Chromatids.
So basically it goes Bases-DNA-Chromosome-Chromatid.