claim vs motion

I would be very grateful if anybody could provide me with any information
regarding the difference between the two documents filed with the court in
the UK, i.e. "claim" and "cause of action". I am currently studying the
British system of judicial proceedings, viz. the documents, filed with the
court. It seems that these two documents are very much similar. However,
legal terminology requires precision, and I just wondered about the
difference.

[Law200]

Claim is often technically what you are demanding etc, while cause of action is the legal reason that allows the claim.

You may demand reimbursement due e.g. to fraud, breach of contract and bad faith--three causes of action.

Thanks a lot! And can a "claim" be used to refer to a legal document which is usually called a "complaint". Most text-books on British civil litigation say that a claimant files a complaint, to which the defendant sends an answer. However, one text-book on judicial proceedings (published 1998) lists such documents as "claim", "particulars of claim" and "defense" without mentioning anything about "complaint" and "answer". Does it mean that in this case "claim" is the equivalent of "complaint", while "defense" - that of "answer"? Sorry if my questions sound stupid, but here in Russia litigation procedures are very different, and I do my best to establish which documents might be in any way equivalent.