One chromosome contains one gene or an allele?Each chromosome is in pair with other chromosome having same type of genes?

Yes you are right in saying that each chromosome occurs in pair. Paired chromosomes are known as homologous chromosomes.
Homologous chromosomes have the same genes arranged in the same order, but they have slightly different DNA sequences. Different versions of the same gene are called alleles. Homologous chromosomes often contain different alleles.

Notice that these two chromosomes are homologous because they have alleles for flower colour at the same position on the chromosome but one allele is for purple flowers and the other for white flowers.