It can also be used more broadly to refer to any written communication, like a message, note, or even an email.
Here are some examples:
* Sumulat ako ng liham sa aking kaibigan. (I wrote a letter to my friend.)
* Natanggap ko ang iyong liham kahapon. (I received your letter yesterday.)
* Nagpadala ako ng liham sa aking pamilya. (I sent a letter to my family.)