what is loop it is difficult to explain without any  diagram.
but i am trying my best to explain it without any diagram 
ok 
firstly understand that what is loop
loop is a cycle.
it is difficult to understand from that explanation.

now let there 3ree switch a , b ,c.which are connected with
each other.now for back up for that connection we provide
alternate path that is called redundancy(suppose that you
know)ok
as if a user want to communicate with another user in to the
network then multiple packet is generated.and these packet
travel through each path within the network.due to this a
loop is formed
diagarm
           a---------b--------c
           <----------------->
lower one is alternate path between a to c.now if a want to
communicate with c then packet travel through both path and
crated loop

now in some cases stp is enabled but loop also formed it is
due to

An STP loop is created when an STP blocking port in a
redundant topology erroneously transitions to the forwarding
state. This usually happens because one of the ports of a
physically redundant topology (not necessarily the STP
blocking port) no longer receives STP BPDUs. In its
operation, STP relies on continuous reception or
transmission of BPDUs based on the port role. The designated
port transmits BPDUs, and the non-designated port receives
BPDUs.   

thanks i think this article may help you
with best regards
jitendera sinha