Cross linking is a link that is exchanged between two domains. For example domain A gives a link to domain B and domain B also links to domain A. That type of bond between two domains is known as a cross linking.
Outbound links are links that are meant to take you elsewhere. These are links that are going to direct you to another specific webpage or website altogether. There are two types of outbound links dofollow & nofollow.
The dofollow link is the normal link, passes on Google PageRank juice from your webpage to the targeted webpage.
A nofollow link is the abnormal link. It does not pass on any Google PageRank juice to the targeted webpage but instead, it blocks off the Google PageRank flow.
Forum posting sites are for sharing your knowledge to others related to your expertise. Form platforms should be used for sharing knowledge. For example this forum is SEO knowledge sharing platform, same forums will be there in your niche,
To find relevant forum platform and share and get knowledge related to your business niche should be a purpose to register in forum sites.
Here is the trick to find forums in Google
Tricks to find guest posting site on search engine.
keyword “submit a guest post”
keyword “guest post”
keyword “guest post by”
keyword “accepting guest posts”
keyword “guest post guidelines”
keyword "guest post"
keyword "guest blogger"
keyword "writers needed"
keyword "blog for us"
keyword "write for us"
For more details, this guide will help you a lot: https://moz.com/ugc/15-smart-ways-to-find-guest-posting-opportunities