What are the differences between Series?

Apple Watch Series 5

• 40mm or 44mm case; over 30 percent larger display
• Always-On Retina LTPO OLED display with Force Touch, 1000 nits
• GPS and GPS + Cellular models
• S5 with 64-bit dual-core processor; W3 wireless chip
• Digital Crown with haptic feedback
• Electrical heart sensor and second-generation optical heart sensor
• High and low heart rate notifications, irregular heart rhythm notification, and ECG app1
• International emergency calling, Emergency SOS, and fall detection
• Water resistant 50 meters
• LTE and UMTS, Wi-Fi, and Bluetooth 5.0
• GPS/GNSS, compass, and barometric altimeter
• 50 percent louder speaker; built-in mic
• 32GB capacity

Apple Watch Series 4

• 40mm or 44mm case; over 30 percent larger display
• Retina LTPO OLED display with Force Touch, 1000 nits
• GPS and GPS + Cellular models
• S4 with 64-bit dual-core processor; W3 wireless chip
• Digital Crown with haptic feedback
• Electrical heart sensor and second-generation optical heart sensor
• High and low heart rate notifications, irregular heart rhythm notification, and ECG app1
• Emergency SOS and fall detection
• Water resistant 50 meters
• LTE and UMTS, Wi-Fi, and Bluetooth 5.0
• GPS/GNSS and barometric altimeter
• 50 percent louder speaker; built-in mic
• 16GB capacity

Apple Watch Series 3

• 38mm or 42mm case
• Second-generation Retina OLED display with Force Touch, 1000 nits
• GPS and GPS + Cellular models
• S3 with dual-core processor; W2 wireless chip
• Digital Crown
• Optical heart sensor
• High and low heart rate notifications and irregular heart rhythm notification
• Emergency SOS
• Water resistant 50 meters
• LTE and UMTS, Wi-Fi, and Bluetooth 4.2
• GPS/GNSS and barometric altimeter
• Built-in speaker and mic
• 16GB capacity (GPS + Cellular); 8GB capacity (GPS)

Apple Watch Series 2

• 38mm or 42mm case
• Second-generation Retina OLED display with Force Touch
• GPS model
• S2 with dual-core processor
• Digital Crown
• Optical heart sensor
• High and low heart rate notifications and irregular heart rhythm notification
• Water resistant 50 meters
• Wi-Fi and Bluetooth 4.0
• GPS/GNSS
• Built-in speaker and mic
• 8GB capacity

Apple Watch Series 1

• 38mm or 42mm case
• Retina OLED display with Force Touch
• S1P with dual-core processor
• Digital Crown
• Optical heart sensor
• High and low heart rate notifications and irregular heart rhythm notification
• Splash resistant
• Wi-Fi and Bluetooth 4.0
• Built-in speaker and mic
• 8GB capacity

If you want a more visual representation of this info, check out this link Apple Watch - Compare Models - Apple