What is the difference between genetic linkage and linkage disequilibrium?

Linkage is the physical proximity of the genes. LD or Linkage disequilibrium is the non random association of the genotype / alleles. The genes contributing to the trait in the case of LD can be physically apart from each other ( sometimes even on different chromosomes )

linkage disequilibrium is the non-random association of alleles at two or more loci, not necessarily on the same chromosome. It is not the same as linkage, which describes the association of two or more loci on a chromosome with limited recombination between them. Linkage disequilibrium describes a situation in which some combinations of alleles or genetic markers occur more or less frequently in a population than would be expected from a random formation of haplotypes from alleles based on their frequencies. Non-random associations between polymorphisms at different loci are measured by the degree of linkage disequilibrium (LD).

Linkage disequilibrium is generally caused by genetic linkage and the rate of recombination; rate of mutation; random drift or non-random mating; and population structure. For example, some organisms (such as bacteria) may show linkage disequilibrium because they reproduce asexually and there is no recombination to break down the linkage disequilibrium.