Phage are the viruses of bacteria. They are also called bacteriophage.
Phage are not alive in the strict sense of the word, they are basically only a lump of genetic material enclosed in a protecting shell, the so called capsid. This "head"
is often geometrically shaped, ie icosahedral or octahedral. Additionally they may have a "tail" structure for the delivery of their payload.
They inject their DNA (mostly - there are some phage consisting of RNA as well) into a bacterium where it reprograms the whole cell machinery to assemble new phage. The cell then splits open and releases the progeny, which usually numbers several hundred. This is called a lytic cycle. Virulent phage enter it at once, temperate phage have the ability to integrate into the genome without killing the host. Thus they are replicated every time the host cell splits. This is termed lysogenic development. The so-called prophage may then begin a lytic cycle later on, when environment conditions worsen and bring about
the possibility of the host cell dying.
Bacteria are not completely defenseless though. They have a kind of immune system, called restriction-modification system. Special enzymes, the restriction endonukleases, cut up all foreign DNA that enters the cell. How do they know it's foreign DNA? Well, because the cell's own DNA is protected by a small modification - it's methylated in a special pattern. Unfortunately some phage imitate that pattern, so there's no complete immunity. But a given phage cannot infect all strains of bacteria - it is host specific.
Apart from that phage are also host specific in the sense that they need certain surface structures to attach. T4 for example can only attack E.coli cells, and some strains of E.coli more efficiently than others because it matches their methylation pattern better.
Phage vary greatly in their complexity. For example ΦX174 has only 5386 nucleotides (single stranded DNA) in 10 genes and a diameter of 25-30 nm, while T4 comes
at a whopping 169000 base pairs (double stranded DNA) in more than 130 genes and is about 300 nm long. It also looks very spacy! See http://www.udel.edu/Biology/Wags/histopage/modelspage/t4.gif :)
As you can imagine, their ability to transfer DNA ("transduction") makes phage a valuable tool in genetic engineering.
Source: Biology of the prokaryotes / ed Lengeler, Drews, Schlegel / Thieme Verlag / 1999