The two platforms that dominate the handheld market are the Palm OS and the Pocket PC. Other options exist, but a comparison of the two main operating systems will familiarize you with handhelds overall.
Palm OS products are very popular, with their easy, elegant interface. Their reputation is that they are easy to set up and learn and have a long battery life. Palms tend to be small, light, and fast on their feet. They use less power than Pocket PCs and have their own well-liked personal information management (PIM) software, including To-Do List, Memo Pad, Data Book, Address Book, Mail, Expense, and Calculator. Palms can synchronize with your PC, and that includes Macs.
The Windows-based Pocket PC comes with a faster chip and more memory than the Palm. Generally, the Pocket PC is larger and heavier too. The Pocket PC may cost more than the Palm, but you'll get what you pay for in power and robust pocket versions of such applications as Outlook (with Calendar and Contacts), Internet Explorer, Word, and Excel. The latest Pocket PCs can work as MP3 players, EbooksTM readers, and digital voice recorders, and they synch up easily with your PC.
Both operating systems have generous software offerings and support from third-party developers, and Palm OS is licensed to third-party handheld manufacturers too.