Tor only carries TCP traffic, which is typically used in applications, such as web browsers and email clients, where reliable and error-corrected transmission is essential. Although the Tor Browser Bundle includes only Firefox, it's possible to configure other applications to use Tor for TCP traffic. As covered in other answers, configuring other applications to use Tor properly is nontrivial. Tails and Whonix are good choices for beginners.
Applications use UDP traffic when speed is more important than reliability and accuracy. Although Tor does not carry UDP, there's a specific work-around for UDP-mediated DNS queries, which translate numeric IP addresses to hostnames (URLS).
Skype and other VoIP apps typically use TCP for a control channel, and UDP for a data channel. Although Skype and Mumble, for example, can fall back to TCP for data over Tor, the error-correction process typically leads to unworkable latency. The typical latency of a Tor circuit is 1-2 seconds. If data loss causes TCP to buffer data during retransmission, the latency can increase two or more fold.
One solution is routing TCP-based openvpn connections through Tor. The openvpn tunnel will carry UDP etc as well as TCP. In my experience, Mumble works very well via VPN through Tor, with high voice quality, virtually no breakup and 1-2 second latency. You can run both a Mumble server, and an openvpn server as a hidden service, in a Whonix instance, and then distribute VPN connection credentials to your contacts. Also, it's more secure than Skype etc because there are no third-party servers involved.
You can also route free VPN privacy services, such as SecurityKISS, through Tor in order to access other UDP-dependent services. However, free VPN services typically limit bandwidth and daily throughput. But you can use non-free VPN services safely if you pay with cash by mail, or with Bitcoins that have been thoroughly anonymized through multiple mixing services.