Dear Deepak,
I'm not a PowerPoint expert so I can't explain things to you in PowerPoint terms. In Keynote, there are themes that are made up of master slides. Master slides act as templates for commonly-used slide formats--an opening title slide, a slide with a title and bullets, etc.--that cover most of the needs for the average slideshow.
Let's say that you are making a slideshow using the "Leather Book" theme. Let's also say that for the "title and bullets" slides in your presentation, you don't want Baskerville for your bulleted text, you want Futura. So under "View" you would choose "Show Master Slides" and you would select the "Title and Bullets" slide. Then you would select the bulleted text and change the font.
Now, hide the master slides and add a slide after the title slide. By default it will be a "title and bullets" slide. If not, just choose the "title and bullets" master from the drop down menu and it will be applied to the slide. When you add bulleted text to the slide, it will be in your new font, Futura.
And, if you have some title and bullet slides in this presentation that were made before the change to the master slide, you can just right-click the slide in the slide chooser and choose "reapply master to slide." It will format the bullets as Futura.
The thing to note here is that you have only changed the "title and bullets" master FOR THIS PRESENTATION. If you were to start a new presentation using the "Leather Book" theme, the font for "title and bullets" would be Baskerville. If you want to use your newly formatted master slides in future presentations, you should save them in a new theme.
All you have to do is make the changes to the master slides that you like and then choose "Save Theme as..." from the File menu. You'll give your new, personalized theme a new name and it will be available for you to use in the future.
Things like footers etc can be easily duplicated across masters. Just put the items you want on one master--say a logo, a footer and some sort of horizontal rule--and select them all. Copy them and then move to the next master you want to format. Paste and everything will be pasted in position.
Changing the color of a slide is a little slower as you have to do it on each master one at a time, but you can add custom colors to the color picker on the bottom and that makes things a little faster.
Let me know if all this makes sense and if you have further, specific questions.