Twilio Segment is an SMS messaging service that acquires the segment size based on character encoding standards and headers used to send messages. Understanding how segments work can help analyze costs and throughput, and crafting the perfect message involves considering encoding standards, data headers, and potential gotchas such as non-GSM spaces and smart quotes. The use of Unicode characters can limit segment size, and using a different character set like UCS-2 may be necessary for certain messages. Twilio's Copilot service now includes Smart Encoding feature to automatically translate Unicode characters into GSM7/ASCII characters.