Here's how Nokia describe the service on their Conversations blog post:
We like to say that Nokia Pulse is smarter messaging. It automatically tags what you send with where you are, and even lets you suggest a place to meet, complete with ratings and reviews, map, directions, and more. You can do this all in one Nokia Pulse message, instead of sending text after text to say where you are and decide where to meet.
There is a lot going on here, but fundamentally this is a messaging application which can be extended through the geo-location services on your smartphone. Nokia are making a big play in location services (for example the updates to Nokia Drive and services such as Nokia Where and City Lens discussed on AAWP last week) and Pulse is another part of that strategy.
That's one reason that the service asks you to login with your a Nokia account. Facebook is available as an alternative, which many people will be expecting and be comfortable with.
Live Tile support has also been improved - if you pin the app to your start screen then this tile keeps a track of unread messages, while you can also pin single threads of messages and conversations to your start screen for quick access.
If the person you're sending the Pulse message to is already using the app, they will get an will alert for the incoming message on their phone, otherwise they'll get the details via email and offered the chance to download the app for their phone. Currently available for Windows Phone, Symbian, and Meego (along with a web based version at pulse.nokia.com), the Pulse website says that a Series 40 version is coming soon, and both iOS and Android will have their own Pulse clients in the Autumn.
With the caveat that this is a beta application, you can grab the updated version from the Windows Marketplace, or via our App Directory.