Diploid organism have double the number of chromosomes as compared to that in haploid organism.
The number of chromosomes is constant for any particular [species](http://socratic.org/biology/the-elements-of-an-ecosystem/species).
Diploid organism has two sets of chromosomes. Different chromosomes in a set of chromosomes have characteristic size and specific position of centromere,
These can easily be differentiated from one-another and are recognized by a specific number, i,e. Chromosome 1, chromosome 2, etc.
The organism with one set of chromosome is termed haploid (n).
Each chromosomes in a set of chromosomes has exactly identical chromosome in other set of chromosomes in diploid (2n) organism. In Pisum sativum (Edible pea), n=7. Somatic cells in Pisum sativum are diploid ( 2n=14). The two chromosomes of the same type in two sets are termed homologous chromosomes.
In higher plants like Spermatophytes, the main plant body is diploid. In lower plants like algae and fungi, the main plant body is haploid.
It may be be concluded that haploid organism has only one set of chromosomes; whereas diploid organism has two sets of chromosomes. Each chromosome in one set has its homologous chromosome in the other set.