A Web Browser
You need to congratulate Microsoft on pulling off this feat: Not only does the PowerPoint Web App work great in Microsoft's own Internet Explorer, but it performs on Mozilla Firefox as well. In addition, support for Apple Safari on the Mac is also provided.
A Windows Live Account
A Windows Live account (or just a Live account) is essentially an email account. You already have a Live account if you have a Hotmail, MSN, or a Live email ID. You can use this Live account to sign in to SkyDrive.com, a file-sharing and storing portal that also provides free access to Office Web Apps. If you don't have a Live account, sign up for one at http://www.live.com. On that page, you will find a Sign In option that also lets you register a new account.
Do you really need to access Office Web Apps through SkyDrive? Unless you have a suitably equipped SharePoint account or a newer Office 365 subscription, your only way to access Office Web Apps is through SkyDrive.comand did I say that SkyDrive was the only totally free option? SharePoint and Office 365 are both paid options; if you have access to them, you'll find that the PowerPoint Web App works similarly to how it works on SkyDrive.