Some viruses (M13) exist as a single strand of DNA while
encapsulated in the protein coat. After they infect a bacterium,
they inject the single strand and create a complementary strand to
it. The complement (the replicative form when double stranded)
creates more copies within the host while the other is packaged in a
protein coat to infect other potential hosts. + and - just denotes
which strand you are talking about.