The basic function of share widgets is to share your Pokemon on the forums easily. You may only display them in your signature and/or on your profile. The widget will show a blue button saying "Train!" if you have not interacted with that Pokemon. Clicking this button will allow the Pokemon to gain experience points. If you have already clicked that Pokemon, the message on the button will say "Thank you!"
There are three different share widgets currently available.
*HTML: Hypertext Markup Language- Used for website coding, wherever supported.
*BBCode: Bulletin Board Code- Coding commonly used for forums, this type will only work on PokeHeroes.
*Facebook: Posts a message on your timeline on Facebook to invite your friends to click your Pokémon.
You can find the coding for your widgets under Community -> Share Widgets.