Recently I have been using Yammer more often than Twitter, but they both have their place. You probably know what Twitter is as it has been going mainstream like crazy the last month or two. Yammer on the other hand is less known. Yammer is sort of like Twitter, but just for your organization.
What Yammer brings, besides being just internal to your place of work, are other enhancements. Groups, for one, is huge. I can create a group say for “Microsoft SQL Server”, and anyone on Yammer in my place of work can join that group and share, discuss, and consume anything from that group. Instead of me sending out emails from articles and tips, people can just subscribe to them.
Also, departments can create groups , private groups, to have discussions just within their dept.
Yammer also has an iPhone app, and a Adobe Air Desktop App (I think I heard TweetDeck integration soon if not already), and it uses SMS message as well if you want, and also you can get daily digest emails from your groups and people you follow.
I would suggest groups of employees that are on Twitter going back and forth, take a look at Yammer for internal needs. What I try to do is save anything for Twitter that is more applicable to everyone, and Yammer for things I just want to share with people I work with.
On that note as well, I separated out my Twitter and Facebook feeds, and am going to try to use Facebook for more non-technical status updates.
Yammer = company/corporate/internal groups
Twitter = colleagues outside of the company, and people I find interesting in my same line of work, etc
Facebook = family, friends, etc