In standard educational terms, a curriculum is an integrated course of study that the teacher follows when preparing lessons for her class. In homeschooling terms, it can mean the same if you are planning your own curriculum; however many curriculum-based homeschooling methods (opposed to unschooling, which is child led rather than curriculum led) make use of pre-packaged curriculum.
This pre-package curriculum often includes everything from an outline of each course to teaching guides, student's texts, worksheets, and testing materials. Of course you can always modify or supplement a pre-packaged curriculum to fit your child's interests and learning style.
Whether using purchased curriculum or your own, you may need to be making sure it fits the state's requirements. If due to an oppressive state homeschooling law, you must be in sync with the public school's curriculum, you can do so rather easily. Your state's department of education probably has a Website where it posts its curriculum for the appropriate grade levels. If not, call your district office and request a copy. Then look at the objectives and goals for each subject and build your curriculum to include them.
In some instances, you may even want school textbooks. Many districts will loan these to homeschooling families, especially if it is one that requires you register your child as homeschooled, because they are likely receiving some funding for homeschooled children.
You can also pick up free textbooks from schools that have changed to a new text. Remember, the "obsolete" stamp on these books generally doesn't mean the text is not a good one. In our old district, they changed from a math program to a "modern" and highly controversial one because a respected school district in another state did so (peer pressure at the administrative level!).
The library is a wonderful, never-ending resource. Also check with your homeschool group for book exchanges. Other resources for curriculum include local bookstores that cater to homeschooling families or teachers. Homeschooling events such as conventions are another resource.
Once you seriously start looking for homeschooling curriculum, you may get overwhelmed. To make decisions easier, first spend some time contemplating your homeschooling goals and looking into state requirements.
Remember that one of the beauties of homeschooling (unless squelched by your particular state law) is that you can choose the curriculum that best fits the needs, learning style and interests of your child as well as your values and even your schedule.
About.com has a page with links to curriculum resources, including a forum where parents discuss and trade curriculum: