"Teu" (and its associated forms) are related to "Tu". If you address someone with "Tu", you'd use teu/tua/teus/tuas to address something they owned.
"Seu" (and its associated forms) are related to "Você" (and also all other third person pronouns, but that's not relevant here). If you address someone with "Você", you'd use seu/sua/teus/tuas to address something they owned.
The ending of the pronoun matches the gender and number of the thing being owned/possessed:
- Teu/Seu - singular masculine noun (teu/seu carro)
- Tua/Sua - singular feminine noun (tua/sua camisa)
- Teus/Seus - plural masculine noun (teus/seus carros)
- Tuas/Suas - plural feminine noun (tuas/suas camisas)