Whe mouse pointer placed on an element I want to show some text. For example if you place your cursor on upvote of an answer, it shows "This answer is useful". You can see it in below image. I did't even know how to search this.

Whe mouse pointer placed on an element I want to show some text. For example if you place your cursor on upvote of an answer, it shows "This answer is useful". You can see it in below image. I did't even know how to search this.

Add a title attribute to the element. That's what's done in the image above.
<a class="vote-up-off" title="This answer is useful.">up vote</a>
If you want a custom tooltip, see this answer. You would basically use a combination of pseudo elements and custom data-* attributes. You would give the pseudo elements a content value of attr(data-tooltip) in order to access the data-tooltip attribute, for instance.