Is the chemical structure of an amide bond (-CONH) or (-CONH2)?
Amides are derivative of carboxylic acids. It consists of 2 parts, and the bond between carbonyl carbon and nitrogen of ammonia is called is amide bond. 

The bond structure of 1° amides is as shown in the figure. 



What you have shown in the question is bond structure of 2° amide, which can be obtained by removing 1 hydrogen from 1° amide. 



Now if you observe structure of nylon, you will see there are n numbers of 2° amides. 




  And also, I see online that lots of bond structures (like NH2 and COOH) are prefixed with a dash, so they'd appear like -NH2 and -COOH. Is this just a personal preference, or is there a more elaborate reason behind this?


The dash means it is Functional Group like -R means hydrocarbyl functional group, -COOH means carboxylic acid functional group, etc.