1. A computer system for determining a set of variation-phrases from a collection of documents, the system comprising: a processor; and a memory storing instructions, wherein the said instructions are executable by the processor to: access a document corpus comprising a plurality of documents; for each document in the document corpus: identify a set of similar documents from the document corpus having at least some tokens in common; align the text of the documents of the identified set of similar documents to compare and identify potential variation-phrase pairs, each identified potential variation-phrase pair comprising two phrases from said identified set that are non-synonymous descriptions of a common item type offered for sale; wherein aligning the text of the documents of the identified set of similar documents to identify potential variation-phrase pairs comprises: aligning the text of a first document from said set of similar documents with the text of a second document from said set of similar documents; and identifying non-matching tokens that resulted from aligning the first and second documents from said set of similar documents as potential variation-phrase pairs; and add the identified potential variation-phrase pairs to a variation-phrase set, at least some of the phrases in the variation-phrase set being non-synonymous with the other phrases in that set; filter the potential variation-phrase pairs to remove from the variation-phrase set potential variation-phrase pairs that do not satisfy one or more predetermined criteria; and store the resulting variation-phrase set in a data store.