An Application Programming Interface (API) is a way for different computer programs to interact with one another. Different computer programs are written in different languages, but often must communicate with one another despite the language barrier. Think of an API as being a door that allows outside programs to interact with the specific program to which a given API applies. As a programmer, if you dont use the door to access that specific program, you aren't getting access.
Short message service (sms) is a standard that allows messages of up to 160 characters -- including spaces -- that is used to communicate with cell phones using the Global System for Mobile Communications (GSM) standard. If you send or receive text messages, you may have noticed that some text messages get broken up into multiple parts. This occurs in compliance with that 160-character SMS standard. Some cell phones offer helpful information when this happens, such as saying Part 1 of 4 for a message thats broken into several pieces. Unfortunately, not all phones do this. A good way to avoid the potential confusion of jumbled multi-part SMS messages is to stick to the 160 character limit.