ATtiny25 2K Bytes of In-System Self-Programmable Flash, 128 Bytes In-System Programmable EEPROM, 128 Bytes Internal SRAM. Two 8-bit timer/counters with PWM and prescaler, 10-bit ADC, USI-Universal Serial Interface. debugWIRE for on-chip-debug. Up to 20 MIPS throughput at 20 MHz. 1.8 - 5.5 V operation.
8KB version: ATtiny85
4KB version: ATtiny45

ATtiny26 2K Flash Program Memory, 128 Bytes SRAM, 128-Byte EEPROM, 11-Channel 10-bit A/D converter. Universal Serial Interface. High Frequency PWM. Up to 16 MIPS throughput at 16 MHz.

ATtiny28L 2K Flash Program Memory, 32 bytes register file, Keyboard interrupt, high-current LED driver, Analog Comparator. Up to 4 MIPS throughput at 4 MHz.

ATtiny44 4K Bytes of In-System Self-Programmable Flash, 256 Bytes In-System Programmable EEPROM, 256 Bytes Internal SRAM. 8-bit and 16-bit timer/counters with PWM, 10-bit ADC, USI-Universal Serial Interface, On-chip Temperature Sensor. debugWIRE for on-chip-debug. Up to 20 MIPS throughput at 20 MHz. 1.8 - 5.5 V operation.
8KB version: ATtiny84
2KB version: ATtiny24

ATtiny45 4K Bytes of In-System Self-Programmable Flash, 256 Bytes In-System Programmable EEPROM, 256 Bytes Internal SRAM. Two 8-bit timer/counters with PWM and prescaler, 10-bit ADC, USI-Universal Serial Interface. debugWIRE for on-chip-debug. Up to 20 MIPS throughput at 20 MHz. 1.8 - 5.5 V operation.
8KB version: ATtiny85
2KB version: ATtiny25

ATtiny84 8K Bytes of In-System Self-Programmable Flash, 512 Bytes In-System Programmable EEPROM, 512 Bytes Internal SRAM. 8-bit and 16-bit timer/counters with PWM, 10-bit ADC, USI-Universal Serial Interface, On-chip Temperature Sensor. debugWIRE for on-chip-debug. Up to 20 MIPS throughput at 20 MHz. 1.8 - 5.5 V operation.
4KB version: ATtiny44
2KB version: ATtiny24

ATtiny85 8K Bytes of In-System Self-Programmable Flash, 512 Bytes In-System Programmable EEPROM, 512 Bytes Internal SRAM. Two 8-bit timer/counters with PWM and prescaler, 10-bit ADC, USI-Universal Serial Interface. debugWIRE for on-chip-debug. Up to 20 MIPS throughput at 20 MHz. 1.8 - 5.5 V operation.
4KB version: ATtiny45
2KB version: ATtiny25

This applies to all types of AVR microcontrollers because they all use same AVR core technology. So lets go through some of them:

Ability to work at 1MIPS/MHz (MIPS – Millions Instructions Per Second);

On chip Flash memory (now tiny series have up to 8kB, mega up to 256kB) with write cycle not less than 1000 times;

On chip RAM memory (tiny series up to 512B, mega up to 8kB);

On chip EEPROM memory (tiny series up to 512B, mega up to 4kB) with write cycle not less than 100000 times;

Working frequencies up to 20MHz;

Protection fuses from unintentional reading and modification of program and data memories;

Ability to program in system – directly on board using ISP cable;

Four types of clock generators: Internal generator, RC generator of clock cycles, Internal generator with external crystal resonator, and External synchronisation;

Two or three power down modes;

Some microcontroller can work at very low voltages like 1.8V;

Other internal processor properties:

Static architecture where minimal frequency equal to zero;

ALU is connected directly to general purpose registers;

Most of commands are performed in one clock cycle;

Multilevel system of interrupts;

Many sources of interrupts – two of them are external;

Three level hardware stack.

I/O system properties:

Software configurable and selection of I/O ports;

Ports can be programmed as inputs and outputs independently to each other;

Input buffers uses Smith triggers on all pins;

Support of Internal Pull-Up resistors (about 35 to 120kOhm);

Peripheral devices vary from device to device. Refer to datasheets to find out what peripherals are included in concrete model. In general we can mention:

8 bit timer counter with pre-scaler;

16 bit timer counter with pre-scaler ;

Watch dog timer;

PWM generator;

Analog comparator;

ADC;

Hardware modulator;

SPI interface;

TWI (I2C) interface;

UART;

For additional information refer to datasheets.