For now there are not enough details about the upcoming iPhones. But it is expected that iPhone 5S will have a design and size similar to that of iPhone 5. It will, however, is likely to be powered by a more powerful processor and may pack in a high resolution screen.
iPhone 5C, which is rumoured to cost around $450, is likely to use a plastic shell instead of the aluminum one on the more expensive iPhone 5S. It is also likely to have slower processor and a screen with lower resolution. Apple hopes to compete with Samsung and other companies making Android phones in developing markets with iPhone 5C.
The Apple invite features colourful bubbles, confirming the rumour that iPhone 5C (and possibly iPhone 5S) will be available in several colours. The invite also notes "This should brighten everyone's day". This is likely to be a reference to iPhone 5C, which means Apple will offer at least two versions of iPhones to meet the different needs of different customers.
This is different compared to Apple's current strategy of offering only one iPhone per generation.