PowerBook 520 (1994)

The PowerBook 520 featured a 25 MHz 68LC040 processor, 4 MB or 12 MB of RAM, and a 160 MB or 240 MB hard drive. The screen was a 9.5-inch grayscale passive-matrix display. 

The PowerBook 500 series laptops introduced the “trackpad” to the Macintosh: the cursor followed the movement of your finger on a pad rather than spinning a plastic trackball with your finger. The trackpad has proven to be a revolutionary input device and has been used since in most notebooks. The PowerBook 500 series also introduced the idea of dual-swappable bays that could be used to hold either one battery and a PCMCIA adapter or two batteries.

Source: EveryMac.com

PowerBook 540 (1994)

The PowerBook 500 series laptops introduced the “trackpad” to the Macintosh: the cursor followed the movement of your finger on a pad rather than spinning a plastic trackball with your finger. The trackpad has proven to be a revolutionary input device and has been used since in most notebooks. The PowerBook 500 series also introduced the idea of dual-swappable bays that could be used to hold either one battery and a PCMCIA adapter or two batteries.

The Macintosh PowerBook 540 featured a 33 MHz 68LC040 processor, 8 MB or 12 MB of RAM, and a 240 MB hard drive. The screen was a 9.5-inch grayscale active-matrix display.

The PowerBook 540 was similar to the PowerBook 520 that was being offered at the same time, but the PowerBook 540 had a faster processor and a higher quality active-matrix display.

Source: EveryMac.com

PowerBook 180 (1992)

The Apple Macintosh PowerBook 180 featured a 33 MHz 68030 processor, 4 MB of RAM, either an 80 MB or 120 MB hard drive, and an internal 1.44 MB floppy drive. The screen was a 9.8-inch grayscale active-matrix display.

The PowerBook 180 supported 4-bit grayscale on the the built-in display, and it allowed 8-bit color when plugged into an external monitor.

Because the laptop supported an external color screen, included a fast processor for the time, and supported a 120 MB hard drive, this PowerBook was the first Macintosh laptop that could replace a desktop with no compromises. In fact, the PowerBook 180 was equivalent in performance to the Macintosh LC III+, also available at the time.

Source: EveryMac.com