how do people hack into your e-mail?

Hi,
I am curious to know how people hack into your e-mail account, and when they do, are they able to access all your e-mails including those with attachments?

The reason I ask is because I had recently enrolled into some courses using my e-mail account, but recalling that my e-mail attachment contains my credit card information. So should I be worried, and if so, what are my next steps to prevent my credit card from being exploited by scammers?

Cheers,
Ice101

If you are worried that your card details are compromised, inform the card company as soon as possible.

Yep, people can hack into anothers email account. I think it mainly happens when one doesn't pay enough attention. Now a days hackers are targeting social networking profiles like facebook, twitter and etcetera.
First of all, someone could simply guess someones password. Although that scenario would be highly unlikely if the password was strong. Another method is called phishing whereby users are asked to enter details at a fake website whose look and feel are almost identical to the legitimate one. The hacker usually sends an e-mail that appears to come from a bank, or credit card company — requesting “verification” of information.
Keyloggers are another method whereby a user clicks on an innocent looking link in an email message.
When you click on that link, the keylogger is automatically installed on your PC without any warning and sometimes antivirus fails to recognize this. Now that the Keylogger is residing in your PC, the hacker can now monitor all the activities on your PC. the keylogger constantly collects all the the keystrokes entered by you in a temporary file, which is then periodically uploaded to the hackers location over the internet. It's like a spyware programme if I could describe it as such.
Hope that helps

Jolly covered the primary ways above pretty well. I will say that phishing (which is really social engineering and not hacking per se) is the most common way for criminals to get mailbox info, passwords, credit card numbers, etc. The simple way to avoid this pitfall is to never type in your private info in an email, or on a website sent to you by email. For example, if you get an email from paypal saying there is a problem with your account, do not follow the link in the email. Instead type in paypal.com, and check your account from there. You can help to fight these criminals by reporting all phishing emails to the people they are imitating (using their abuse address if they have one) so they can warn their customers and disable any fakes. For example, fake paypal emails should be forwarded to [email protected].

As far as hacking and viruses go, make sure your computer always has up to date virus protection, and your software (operating system, browsers, even software like word that can run macros) has all current security updates. Make sure your home network is secure, and use extreme caution when connecting to unsecured wireless networks. Run a full virus scan periodically. Make sure your passwords are difficult to crack (use letters and numbers, don't use your birthdate or other info someone might have access to). These precautions will greatly reduce the likelihood of your information being compromised.

Thank you for the responses! They are very helpful.