How do credit card companies know of fraudulent activity?

I have had 2 cases with credit cards over the past year where the company contacted me about fraudulent activity even before it was listed on my account. One time I was asked about a charge from WalMart in another state. I haven't even used these credit cards for months before they asked me about fraud charges on my cards. How does someone get my credit card numbers and how does the company suspect that it is fraud and it wasn't my purchases?|||The banks look for unusual patterns of card use.