After a ship is officially christened it needs to be fitted. In the past this could mean just bringing on the stores needed for a voyage and then the ship was ready for use. Today fitting out can mean another year or two in the yards.

In the early days of the US fitting out and commissioning had little or no ceremony.
Take for example the orders received by Captain Thomas Truxtun in 1798:

“Sir, I have it in command from the president of the United States, to direct you to repair with all due speed on board the ship Constellation lying at Baltimore. It is required that no Time be lost in carrying the Ship into deep water, taking on board her Cannon, Ammunition, Water, Provisions & Stores of every kind completing what work is yet to be done shipping her Complement of Seamen and Marines, and preparing her in every respect for Sea . . . It is the President's express Orders, that you employ the most vigorous Exertions, to accomplish these several Objects and to put your Ship as speedily as possible in a situation to sail at the shortest notice.” Source - NAVAL HISTORICAL CENTER

During World War II some ships had a fit out and sea trial period lasting 20 days. The USS Monitor was commissioned just two weeks after she was christened. Today the newest carrier might take 3 years before it is commissioned.

Fitting out includes finishing all the ships systems. Adding the ships armament and ammunition. Loading her with needed stores for operation and loading all vital fluids, fuel, water, and lube oil.

For example LCS2 USS Independence was christened on October 8, 2008. When I saw her a month later she was outside her graving dock in the water. She is set for delivery to the Navy in early 2009. Where as the USS George Bush CVN-77 was christened in October 2006 and will be commissioned in January 2009.

Some ships are ready to hit the water running while other need to be finished. Think of it as moving into your house a living before the drywall is hung, taped, sanded, and painted.