Lync 2010 provides the web meeting/ conferencing/ IM/ chat function for Office 365. You can have meetings with others in your organization, and/or anyone outside of your organization.
If you are planning to join a Lync session, do not have Lync 2010 installed, and you want to use Video/ Audio, then it is best to download and install the Lync 2010 Attendee. The advantage with the Attendee software is that remote users can join using video and audio.
If you have a subscription to Office365 that includes Lync then you should download and install the Lync Client for your local machine. It will allow you to see who in your group is available, setup instant chat or meetings, or even call them via your computer. Here are the links to the Lync Installer for the 32 and 64 bit O/S.