what can attackers do between two end hosts I have one question that I couldn't come up with a nice solution.

So let's have this analogy, I have a box full of money and I want to deliver to this my friend. Here if we think of a potential attacker there are two possible ways, first the attacker would be between us , so called postman or he could pretend like he is the recipient. 

This being the case, what are the properties of both type of attackers?

For the first attacker in the role of the postman, he could simply observe, modify or block the delivery of the box(message in cryptography). And my question here is what could he do if he disguises as a receiver unlik the postman attacker?

and what other kind of attacks could these both attacker types do?
Attacks like this can be prevented by TLS.
Quote from Wikipedia:

The TLS protocol aims primarily to provide privacy and data integrity between two or more communicating computer applications.2:3 When secured by TLS, connections between a client (e.g., a web browser) and a server (e.g., wikipedia.org) have one or more of the following properties:

The connection is private (or secure) because symmetric cryptography is used to encrypt the data transmitted. The keys for this symmetric encryption are generated uniquely for each connection and are based on a shared secret negotiated at the start of the session (see TLS handshake). The server and client negotiate the details of which encryption algorithm and cryptographic keys to use before the first byte of data is transmitted (see § Algorithm below). The negotiation of a shared secret is both secure (the negotiated secret is unavailable to eavesdroppers and cannot be obtained, even by an attacker who places themselves in the middle of the connection) and reliable (no attacker can modify the communications during the negotiation without being detected).

The identity of the communicating parties can be authenticated using public-key cryptography. This authentication can be made optional, but is generally required for at least one of the parties (typically the server).

The connection is reliable because each message transmitted includes a message integrity check using a message authentication code to prevent undetected loss or alteration of the data during transmission.