Apple Watch Versions:

1. Apple Watch (1st Generation)

Release Date: April 2015

Also known as "Series 0"
38mm and 42mm sizes
Only available in aluminum and stainless steel finishes
Single-core S1 chip
18-hour battery life
Limited functionality (no GPS, only works with an iPhone)

2. Apple Watch Series 1

Release Date: September 2016

Same design as the first generation, but with the S1P dual-core chip (for better performance)
Still only available in aluminum
No GPS, just iPhone dependency
Improved battery life

3. Apple Watch Series 2

Release Date: September 2016

GPS built-in (first version to feature this)
Water resistance up to 50 meters (swimming-friendly)
Brighter display
S2 dual-core chip
Ceramic and aluminum options, with new color choices

4. Apple Watch Series 3

Release Date: September 2017

Cellular connectivity option (first Apple Watch to support LTE)
S3 dual-core chip for faster performance
Barometric altimeter for elevation tracking
Available in aluminum and ceramic (ceramic only for the cellular model)

5. Apple Watch Series 4

Release Date: September 2018

Larger displays (40mm and 44mm, up from 38mm and 42mm)
ECG (electrocardiogram) app for heart health monitoring (first in an Apple device)
Fall detection feature
S4 64-bit dual-core chip for better performance
Improved design with slimmer body and more rounded edges
Taptic Engine improvements

6. Apple Watch Series 5

Release Date: September 2019

Always-On Retina display (the screen is always visible without having to raise your wrist)
Built-in compass
S5 64-bit dual-core chip
International emergency calling (via cellular model)
Available in new materials, including Titanium and Ceramic

7. Apple Watch Series 6

Release Date: September 2020

Blood oxygen sensor for oxygen saturation monitoring
S6 dual-core chip for faster performance
Always-on display brightness increased by 2.5x
New color options, including blue and red
Family Setup feature to set up Apple Watches for family members without an iPhone

8. Apple Watch SE (1st Generation)

Release Date: September 2020

A more affordable version of the Series 6
Lacks blood oxygen sensor and always-on display
Powered by the S5 chip from the Series 5
Same size and design as the Series 6
Available in aluminum only

9. Apple Watch Series 7

Release Date: October 2021

Larger display (41mm and 45mm sizes) with thinner bezels and more rounded edges
Faster charging (about 33% faster than Series 6)
IP6X dust resistance (first in an Apple Watch)
Blood oxygen sensor and ECG
S7 chip, offering improved performance and power efficiency

10. Apple Watch SE (2nd Generation)

Release Date: September 2022

Similar to the first SE but with a faster S8 chip (same as the Series 8)
Lacks advanced health features like ECG and blood oxygen sensor
Available in 40mm and 44mm sizes
More affordable than Series 8

11. Apple Watch Series 8

Release Date: September 2022

Temperature sensor for cycle tracking and early detection of fever
Crash Detection (detects severe car crashes)
S8 chip for improved performance
Always-on display and improved battery life
IP6X dust resistance and WR50 water resistance
Available in new colors and materials, including aluminum, stainless steel, and titanium

12. Apple Watch Ultra

Release Date: September 2022

A rugged, larger 49mm case aimed at outdoor adventurers
Dual-frequency GPS for more precise location tracking
Longer battery life (up to 36 hours, up to 60 hours with low-power mode)
S8 chip
Titanium case for durability
Features like Depth Gauge, Oceanic+ app for divers, and 100-meter water resistance

13. Apple Watch Series 9

Release Date: September 2023

S9 chip (faster processing and better energy efficiency)
Enhanced performance for Siri and health features
Double Tap gesture for controlling the watch with your fingers (without touching the screen)
Improved U1 chip for precise location and tracking
New color options and recycled materials in construction

14. Apple Watch Ultra 2

Release Date: September 2023

S9 chip (improving processing, battery, and user experience)
Ultra-wideband improvements for more accurate tracking and integration with the U1 chip
Upgraded display with better brightness and efficiency
Longer battery life, with up to 72 hours in low-power mode
More eco-friendly with a greater focus on recycled materials

Summary of Key Features Evolution:

Design & Display: Apple Watch models have evolved from a small and basic design to larger, edge-to-edge displays with thinner bezels, and better display technology (like Always-On displays).
Health & Fitness: Over time, health features have expanded, adding ECG, blood oxygen monitoring, temperature sensors, and fall detection. The Series 6 introduced a blood oxygen sensor, and the Series 8 added temperature sensing and crash detection.
Battery & Performance: The newer models have faster chips, better battery efficiency, and quicker charging speeds, culminating in the Ultra series offering multi-day battery life for extreme users.
Durability: The Ultra models introduced rugged features for extreme conditions, with more advanced GPS and dive tracking.

Each new generation of the Apple Watch builds on the features and innovations of the previous one, refining both its design and functionality, especially for health, fitness, and connected lifestyle features.