Could you please explain the difference between contract and agreement? Is there actually any difference between these two terms in American/British legal system? For example in the text below the terms are expressly distinguished.

«It was all handled according to numbers,» Virginia said. «General classifications. For instance, number one to a thousand was personal correspondence. Number one thousand to three thousand represented contracts. Three thousand to five thousand, probate. Five thousand to six thousand, wills. Six thousand to eight thousand, agreements. Eight thousand to ten thousand, real estate transactions.»
by Erle Stanley Gardner "The Case of the Horrified Heirs"

Thank you in advance
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Comments  (Page 6) 
offer + acceptance + consideration + legal obligation = contract
offer + acceptance + consideration = agreement

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