What are the differences between carriers and freight forwarders?

Definition of Carrier in International Conventions:

Global sea cargo transportation is governed by international conventions.

The first convention signed in international scale related to international sea transportation is known as "Hague Rules".

The full name of this convention is "International Convention for the Unification of Certain Rules of Law relating to Bills of Lading and Protocol of Signature".

Hague Rules drafted in Brussels on 25 August 1924 and defines the carrier as follows:

"Carrier" includes the owner or the charterer who enters into a contract of carriage with a shipper.

Definition of the carrier is very straight forward in Hague Rules.

In order to be named as a carrier under Hague Rules you have to be the ship owner or at least the charterer and you have to enter into a contract of carriage with the shipper.

Hague Rules amended in 1979 and new amended rules are known as "Hague-Visby Rules". The definition of the carrier stayed unchanged in "Hague-Visby Rules" as it was in "Hague Rules". 

The second convention signed in international scale related to international sea transportation is known as "Hamburg Rules". The definition of the carrier has been changed significantly with the Hamburg Rules

Hamburg Rules make the distinction between the carrier and actual carrier. 

According to the Hamburg Rules you do not need to own a vessel in order to be defined as a carrier. 

Actually any person or entity who signs the contract of carriage with the shipper is accepted as a carrier under the Hamburg Rules

The party, who makes the sea transportation with its own or chartered vessels are defined as an actual carrier under the Hamburg Rules

Below you can find carrier and actual carrier definitions from under the Hamburg Rules
"Carrier" means any person by whom or in whose name a contract of carriage of goods by sea has been concluded with a shipper.
"Actual carrier" means any person to whom the performance of the carriage of the goods, or of part of the carriage, has been entrusted by the carrier, and includes any other person to whom such performance has been entrusted.
Differences Between Freight Forwarders and Carriers:

Freight Forwarders: Freight forwarders are carriers as defined in Hamburg Rules. They do not need to own any vessel. They sign the contract of carriage with the shippers and then they find actual carriers to perform the transportation. Freight forwarders are known as "contractual carriers" in some occasions.

Carriers: Carriers are performing "Actual carrier" duties as described in Hamburg Rules. They transports goods from port of loading to port of discharge under their own or chartered vessels.