Hybridization is the reaction by which pairing of the complementary strands of nucleic acid happens. DNA is usually double-stranded, and when these strands are separated form each other they will re-hybridize under the suitable conditions. Hybrids can form between, DNA-RNA, DNA-DNA or RNA-RNA. They can form between a short strand and a long strand containing a area complementary to the short one. Imperfect hybrids can also form, but more imperfect they are, the less stable they will be (the less likely to form). To "anneal" two strands is similar as to "hybridize" them.