What is custom 404 page and why we are recommending to set it up?
If someone tried to reach your webpage with wrong spelled file name (www.domain.com/aobut.php), then he will get a default 404 page which states there is no page like that. This may leads user to leave the website.
These visitors may help us to make conversion in the website! Why should we miss them?

For that we can design a custom 404 page for the users who have reached 404 page of our website by misspelled file name. In that 404 page we can give some attractive images and useful links and asking them to get into the links to navigate the website.
If some user try to reach the “about us” by typing the URL www.domain.com/aobut.php (misspelled file name) directly in browser, then it will take him to the custom 404 page www.domain.com/404.php. There he can find the important links of the website which helps him to navigate the website.
How to set it up for the website present in Linux?
1) Design a page with pleasing words, attractive images and important links.
2) Save the file in the root of your server with the name “404.php (any name)”
3) Add the following line “ErrorDocument 404 /404.php” in your htaccess file.
Now all misspelled file names will take user to the 404.php
How to set it up for the website present in windows?
1) Design a page with pleasing words, attractive images and important links.
2) Save the file in the root of your server with the name “404.php (any extension)”
3) Open your hosting control panel.
4) Click on the website link for which you want to set 404 page.
5) Select “Web Services” for that website. (Many of the hosting account is of similar type.)
6) Then click on “Add Error Document”. Select 404 and give your document name (404.php) in the text box.
Now all misspelled file names will take user to the 404.php
Sample: I have given below the custom 404 page of the website which admired me most. I hope you also will be impressed by this. Click Here

A very informative document about setting up a ErrorDocument in your .htaccess file. Thanks a lot.
Nice Article
)))
Thanks for the link
Great Stuff…!!! Clear & concise post about setting up custom 404 pages in windows and linux. Keep up the good work…
DAESH ONOTOLE V PRAVITELI VSELENNOI!
Nice site, thanks for information!
Not bad… Not bad.
All promises are either broken or kept
Yes I agree with you!