These are typically gaps between dwellings in a street scene which otherwise consists of more regularly spaced buildings - rather like a missing tooth in a smile. Generally wide enough for one or two additional dwellings, these are usually classed as one of two types:

Spare land:  Land with no current use, often hidden from view behind walls, hedges or fences.   

Garden plots:  As the name suggests, this is land that currently (or previously) forms part of a property's garden but is sufficiently wide to be separated from the existing property to form a plot for one or more additional dwellings. Previously classed as brownfield land but more recently re-designated as greenfield. 

Rather than seeking to fill the gap in a street scene like an infill plot, a backland plot (which is another type of garden plot) can often be created at the rear of an existing property. These are generally possible where a homeowner has a large rear garden which is of sufficient size to be able to provide space for one or more dwellings, without encroaching on the privacy of the existing donor property. Access to backland plots is normally from the front past the existing property, unless a rear access already exists.  

This is a plot of land which already has an existing dwelling but the structure is of a substandard or dilapidated nature. Alternatively,  the existing dwelling may be in good condition but is very small when compared with the size of the plot it sits on.  In either case, it may be financially viable to knock down the existing structure and build either one or possibly multiple new homes.

This is land which was used previously for commercial or industrial purposes but is now vacant or suitable for being re-purposed for residential dwellings, following the removal of the existing structures and any appropriate de-contamination, etc.  

Retail and other commercial premises can sometimes be fully or partially converted to residential dwellings. Through the use of suitable planning applications, combined with permitted development rights where they exist, lower value commercial space can be transformed into much higher value residential living space.