Question: What do restriction endonuclease enzymes do?
About the article "Virus", section "Bacterial viruses".

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There are numerous types of restriction enzymes, each of which will cut DNA differently.
Most commonly-used restriction enzymes are Type II restriction endonuclease (See article on Restriction enzymes for examples).
There are some that cut a three base pair sequence while others can cut four, six, and even eight.
Each enzyme has distinct properties that determine how efficiently it can cut and under what conditions.
Most manufacturers that produce such enzymes will often provide a specific buffer solution that contains the unique mix of cations and other components that aid the enzyme in cutting as efficiently as possible.
Different restriction enzymes may also have different optimal temperatures under which they function.
Note that for efficient digest of DNA, the restriction site should not be located at the very end of a DNA fragment.
The restriction enzymes may require a minimum number of base pairs between the restriction site and the end of the DNA for the enzyme to work efficiently.
This number may vary between enzymes, but for most commonly-used restriction enzymes around 6-10 base pair is sufficient. 
Homing endonucleases differ from Type II restriction enzymes in the several respects:  Whereas Type II restriction enzymes bind short, usually symmetric, recognition sequences of 4 to 8 bp, homing endonucleases bind very long and in many cases asymmetric recognition sequences spanning 12 to 40 bp.
Homing endonucleases are generally more tolerant of substitutions in the recognition sequence.
Minor variations in the recognition sequence usually decrease the activity of homing endonucleases, but often do not completely abolish it as often occurs with restriction enzymes.
Homing endonucleases share structural motifs that suggest there are four families, whereas it has not been possible to determine simply recognisable and distinguishable families of Type II restriction enzymes.
Homing endonucleases act as monomers or homodimers, and often require associated proteins to regulate their activity or form ribonucleoprotein complexes, wherein RNA is an integral component of the catalytic apparatus.
Type II restriction enzymes can also function alone, as monomers or homodimers, or with additional protein subunits, but the accessory subunits differ from those of the homing endonucleases.
Thus, they can require restriction, modification, and specificity subunits for their action.
Finally, homing endonucleases have a broader phylogenetic distribution, occurring in all three biological domains—the archaea, bacteria and eukarya.
Type II restriction enzymes occur only in archaea, bacteria and certain viruses.
Homing endonucleases are also expressed in all three compartments of the eukaryotic cell: nuclei, mitochondria and chloroplasts.
Open reading frames encoding homing endonucleases have been found in introns, inteins, and in freestanding form between genes, whereas genes encoding Type II restriction enzyme genes have been found only in freestanding form, almost always in close association with genes encoding cognate DNA modifying enzymes.
Thus, while the Type II restriction enzymes and homing endonucleases share the function of cleaving double-stranded DNA, they appear to have evolved independently.  
In molecular biology, the BsuBI/PstI restriction endonuclease family is a family of type II restriction endonucleases.
It includes BsuBI and PstI.
The enzymes of the BsuBI restriction/modification (R/M) system recognise the target sequence 5'CTGCAG and are functionally identical with those of the PstI R/M system. 
In molecular biology, the restriction endonuclease BsobI/AvaI family of enzymes includes the AvaI and BsoBI restriction endonucleases from "Anabaena variabilis" and "Bacillus stearothermophilus", both of which recognise the double-stranded sequence CYCGRG (where Y = T/C, and R = A/G) and cleave after C-1. 
In molecular biology, the DpnII restriction endonuclease family is a family of restriction endonucleases which includes DpnII from "Diplococcus pneumoniae".
These enzymes recognise the double-stranded DNA unmethylated sequence GATC and cleave before G-1, where it encompasess the full length of the protein.