Threads with New PostsWhen you view the forum the column of icons on the left shows which boards contain new posts - the actual image depends on your theme but for example:

shows that there are new posts in this board;

shows that there are no new posts in this board.
You can either click on that graphic, which will display all threads with new posts since your last visit, or click on the board name to show all the threads.
New PostsWithin a board, threads with new posts in them will show

(or similar graphic).
Either click the graphic to have the thread scroll down to the first post you haven't seen, or click on the title of the thread to display all the posts.
New PicturesIf there are new pictures in the gallery it will show on the menu as a number after 'Media' - the number represents how many new items there are. Highlight the Media menu item and click "Unseen" - this shows all the pictures you have not seen! To reset it back to 0 click the button "Mark all as seen" over on the left. When someone next adds a picture a number will appear on the menu again and clicking "Unseen" will show them.
New 'Hot Poop' pagesFrom the menu click Hot Poop. On the left, in the box above the search, click 'Recent Changes' which will show all new and changed pages over the past week.
New LinksForom the menu click Links. From the menu within the links system click "What's New" which will show all links created since your last visit.