Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
"Superinfection" is when a cell, already infected by one virus particle's attack, becomes infected by a second virus. (This could be a substantially different virus from the first, or it could even be exactly the same strain.) Superinfection commonly occurs, even in healthy people infected with benign viruses, but in some cases the infected cell is modified in such a way that secondary infection does not occur.
HIV-1 superinfection has been experimentally demonstrated in chimpanzees, but many have assumed that such reinfection could not occur in previously infected human beings. "Co-infection" has definitely occurred - with two different clades of HIV-1, or with HIV-1 and HIV-2 - but it was always possible that these were due to two viruses infecting at the same time.
On balance of probability, we might expect superinfection (one infection, then another one later on) to be more likely than simultaneous infection (two infections at exactly the same time). It needs to be proved, rather than guessed at, of course!
Superinfection is a dangerous occurrence because it can lead to viral recombination - i.e. two different HIV strains, within a single human cell, could swap genetic information through the process called "recombination" and produce novel strains of HIV. (Recombination is the same process that causes a mother and father's genetic information to be mixed up in their offspring.) Some of these novel strains of HIV could be more virulent, or could be resistant to drug therapies, or could add to the rate of AIDS progression.