positive and negative feedback
A feedback loop is a network structure in which the gene is able to regulate itself via a single path.

$A\rightarrow B\rightarrow C\rightarrow A$

Interactions can be called positive or negative depending on their effect on the downstream gene. If a gene A represses another gene B, then it is a negative interaction; when it activates then the interaction is positive.

If you multiply the signs of all the interactions in the path from gene-A back to itself you will get the sign of the loop. 

So a positive FBL will result (in the above example) if :


A activates (+) B; B activates (+) C; C activates (+) A
A represses (-) B; B activates (+) C; C represses (-) A
A activates (+) B; B represses (-) C; C represses (-) A
A represses (-) B; B represses (-) C; C activates (+) A


Any other combination of interaction will result in a negative feedback loop.

Differences are based on their properties. I'll mention two main unique properties of each.

PFBL


Switching circuits
Hysteresis


NFBL


Expression control (feedback control)
Oscillations


Similarity: well, nothing more that they both are called feedback loops and the topology is similar (just the nature of interaction is different).