If you are using self-hosted WordPress, i.e. that you downloaded yourself via wordpress.org, and installed yourself on the server account provided by your hosting provider, then: yes; you can FTP into your WordPress installation (and in fact, almost have to do so, in order to install it).
If you are using the hosted blogging service wordpress.COM, then: no, you cannot FTP into your account, regardless of whether you have purchased a domain that maps to your wordpress.com account. In this case, you’re still using the hosted service from wordpress.com.