H & C Applications are Permanent Residency applications that are filled from within Canada. You may be able to qualify for an H & C and obtain Canadian permanent residence status. Individuals without legal status would be eligible for H & Cs if they have established themselves in Canada.
A good candidate for an H&C application would be someone who has resided in Canada for a number of years and is unable to qualify under any other category for immigration.
One of the major factors in deciding the H and C grounds is undue hardship which you will be facing when you return back to your home country. Its important to note that H & C applications are not refugee applications.