Perfect Competition:
Perfect Competition means where all firm produces homogeneous product and consumer have full information about the product.
Market structure characteristics of perfect competition:
Number and size distribution of sellers:      Many small sellers.
                                                                No seller is able to exert a significant influence over price.
Number and size distribution of buyers:      Many small buyers.
                                                                No buyer is able to exert a significant influence over price.
Product differentiation:                    Product undifferentiated
Conditions of entry and exit:                 Easy to entry and exit.
                                                                Resources are easily transferable among industries.
Monopoly:
Monopoly refers to a situation where a single firm serves an entire market for a good which have no close substitutes.
Examples:
Courier service expenses, post office expenses are the example of monopoly market.
Market structure characteristics of monopoly:
Number and size distribution of sellers:      Single sellers
Number and size distribution of buyers:      Unspecified
Product differentiation:                    No close substitutes
Conditions of entry and exit:                 Easy prohibited or difficult