The Topologically Orientented Protein Structure (TOPS) project started in 1999 by Westhead et al to capture the overall shape - or ‘fold’ - of a protein. My involvement was in the mid-2000s, summarised in the 2004 paper (see below).
Proteins have a wide diversity of different shapes, to afford an equally large number of biological functions. However, the fold of a protein is nearly always shared between evolutionarily related proteins. A TOPS pattern, is a description of the fold that can be compared to other such patterns.
This allows researchers to both describe a fold in a compact notation and to search or compare folds. Further, a hierarchy of folds can be described - like CATH or SCOP among others.