There are two options for me to save images in Photoshop. One is save for web feature and other is save as. I am developing a site. From these two options which is most suitable for me? 
If these images are to be used on a website, use Save for Web. That feature strips out all the unnecessary stuff like metadata, making the image file much smaller. Save As is for when the image will be used in something like a layout program, where file size is not an issue. You definitely don’t want to be posting Photoshop images on the web withut preparing them for the web, as the file sizes are way too large.
Save for Web is usually if you have sliced your image up for use in an HTML format.
This is what people did back in the day when HTML tables were used to code web sites. It is not common anymore because of HTML5 and CSS of course.
Slicing is indeed a bit out of date now that table layouts have died, but any image that is used on a website should be saved for the web.