Basic difference between Polyclonal and Monoclonal antibodies

Monoclonal Antibodies:

1. The antibody comes from a single cell clone (Monoclonal).
2. These are epitope specific i.e., they can bind to only one epitope on an antigen.
3. Staining by using them gives less background as they have only specific antibodies.
4. They are produced artificially by animal cells in tissue culture which are known as hybridomas.
5. The production is well controlled hence, the antibodies don't have batch to batch variability

Polyclonal Antibodies:

1. The antibody has been produced by many clones (Polyclonal).
2. These are not epitope-specific i..e., they can bind to multiple epitopes of an antigen.
3. Staining by using these antibodies might sometimes produce background signal due to presence of non-specific antibodies.
4. They are derived from animal blood serum which were injected with antigen. 
5. These antibodies might have batch-batch variability.

Due to several of above reasons, monoclonal antibodies are more expensive compared to polyclonal antibodies.

Important note: Epitopes (known as antigenic determitant)

Normally we learn that antibodies are produced against antigens but you need to understand that antigens have several epitopes and antibodies are produced against these epitopes of an antigen but not the whole antigen. As polyclonal antibodies are produced in animals by injecting antigen, antibodies are produced against several epitopes of injected antigen, hence when collected they have antibodies which are antigen- specific but not epitope-specific. The concept of monoclonal and polyclonal becomes clear when we understand this difference. To make more sense, please see the figure below