What is the difference between a forgery attack and an user impersonation attack? I'm new to cryptography, so I was searching for a forgery attack, but unable to find anything. 

Can anybody tell me the difference between forgery attack and user impersonation attack?
Forgery and impersonation attacks are different. Forgerty attacks target on verifiability where impersonation attacks target on authentication. However, they are somewhat linked, as detailed below.

In cryptography, many schemes such as digital signature, message authentication code, verifiable secret sharing etc. require verifiability. 

In such a scheme, a user has a key pair $(k,vk)$ ($k$ and $vk$ can be the same or different). There is an algorithm $f(m,k)$ that takes a piece of data $m$ and the secret key $k$ as input, and output some cryptographic data $c$, and there is another algorithm $v(m,c,vk)$ that takes data $m$ and $c$, and the verification key $vk$, and outputs 1 if $c$ is indeed generated using $f$ from $m$ and $k$, and outputs 0 otherwise.

Forgery attack in this context means an attacker who has no knowledge of the secret key $k$ can generate a pair $(\bar{c},\bar{m})$ such that he has never seen $f(\bar{m},k)$ before, and $\bar{c}$ can pass the verification, i.e. $v(\bar{m},\bar{c},vk)=1$.

A forgery attack can (but not necessarily) lead to an impersonation attack. An impersonation attack is when an adversary can successfully claim an identity that he doesn't possess, and pass authentication (i.e. he can pretend to be another person in the system). Many authentication protocols rely on verifying some cryptographic data, if the cryptographic data can be forged, then authentication will admit the wrong person. 

One prominent example is many authentication protocols rely on PKI, and require the party to be authenticated to provide a valid certificate, and the validation of the certificate relies on much on the CA's signature on the certificate. If one can forge the CA's signature, he can forge a certificate, thus be authenticated as another person.