Microsoft PowerPoint (versions 2010 or later) comes with an in-built feature for recording voice narration, which can be saved within the PowerPoint file or exported as a "PowerPoint Show" or an MP4 video. There are some advantages of using this feature (as opposed to using screen recording software to directly record a video while giving a presentation):
The size of the PowerPoint (.pptx) or the PowerPoint Show (.ppsx) file including the narration is very small, as it includes only the slides and the audio -- much smaller than a normal video file. The .ppsx file can be distributed to students and it plays just like a video (it can be played using PowerPoint or free alternatives).
Narrations are stored separately for each slide. It is easy to insert, delete, or modify slides or their narrations in the middle of the PowerPoint without affecting the narrations on the other slides. This makes it convenient to update content from time to time.
The steps below are demonstrated using PowerPoint version 2013. For Office 365 and other versions, please see this guide on Microsoft website.
Click on the "Slide Show" tab in the top navigation bar. Keep the "Play Narration", "Use Timings", and "Show Media Controls" check-boxes ticked.
Click on the little black triangle on the "Record Slide Show" button. It will open a menu, that has three options:
Start Recording from Beginning
Start Recording from Current Slide
Clear
Click on on the second option "Start Recording from Current Slide"
A popup window will appear as shown in the image. Keep all options checked, and press the "Start Recording" button.
This starts your presentation and anything you speak now into your mic will be recorded. If you have animations in your slide, they will also be recorded with the same timing with which you play them here. You can also go to the next slides and continue the recording for those slides. You can pause or stop the narration using the small navigation menu show in the top-left corner of the slideshow. Narrations will be saved for whichever slides you cover before stopping.
If you would like to change the narration on a particular slide later, go to that slide, open the "Record Slide Show" menu as shown in Step 2, and then select "Clear Narration on Current Slide". After this, you can record the narration again on this slide as shown in previous steps, and then stop after the narration for this slide is recorded.
After recording, you can save your PowerPoint file in any of these formats:
.pptx (standard PowerPoint format): The narrations can be played by just playing the slide show.
.ppsx (PowerPoint Show format): These files open by default in slideshow mode rather than edit mode. Good for sharing with students.
.mp4 video: You can export your slides with narration as an MP4 video for uploading on YouTube etc. Click on "File" menu on top-left corner, then click on "Export" as shown in the image, and press "Create Video" button.
You can also use the in-built pointer/markers for highlighting content on the slides while you are narrating -- these pointer/marker movements will also get saved in the narration. For more features and usage guidelines, please see this link on Microsoft website.
Please also see the video created by Mr. Gopesh Tiwari (from IITK Computer Centre) on how to make narrated lectures using PowerPoint.