What is a chromosome?
Would it be correct to say that before the chromosome divides, there are simply 23 of them?


No, in a normal, non-dividing cell of the human body there are 46 chromosomes in the cell's nucleus. The 46 chromosomes are separate, but are often considered as 23 pairs of chromosomes.  The two chromosomes of a pair are called homologous chromosomes because they contain very similar, but not quite identical, information.  One member of each pair comes from the mother and one from the father.

Each of these non-dividing chromosomes effectively consists of a single chromatid, but because there is only one, indistinguishable from the chromosome as a whole, it is not talked about.  


  Why not just describe the chromosome before it replicates for simplicity sake.


It is often described in that form (as one very long molecule of double-helix DNA).  However, a chromosome is also very commonly illustrated in the mid-replication form where it has two chromatids and takes an X-like shape.  One  reason for this is that chromosomes can be seen in a light microscope only in this mid-replication form, and that is the form in which they
were first observed and first shown to be the basis of heredity (long before they were known to be composed of DNA).