Here's the system card.

Some quick takeaways (from a non-expert) 

• They merged the models and deprecated old ones.
• It seems to choose whether it wants to use a reasoning model for you.
• Seems to be a lot faster and have a lot less hallucination
• You can now use voice mode with reasoning models/the frontier.
• And new personalities (Cynic, Robot, Listener, Nerd)
• Seems like big gains for developers (integrating GPT into their apps etc.)
  
  From Wired:

[Altman called it]  “a significant step along the path to AGI”

Two new model variants: a lightweight gpt-5-mini and an even faster, cheaper, gpt-5-nano (which is only in the API).

OpenAI says that the $200 a month Pro tier offers unlimited GPT-5 access, along with GPT-5-pro, a more powerful version of the model, and GPT-5-thinking, which allows the model to process a query for longer than usual. Pro users will still have access to pick through legacy models.

Starting next week, Pro users will be able to connect their Gmail, Google Contacts, and Google Calendar to ChatGPT, with other tiers gaining access at an unspecified date.