You can use a datafeed to add categories, products (including their discription and images) and prices to the website. You can get a product datafeed from a merchant/online shop.

A datafeed is a text file which contains information about categories, products and prices offered by the merchant. It is the snapshot of the merchant/online shop's inventory. It contains name of product, its category, its price and so on which is separated by some standard character called delimiter. Delimiter can be a comma, pipe, Tab. Each line of the file represents information about a single product.

A datafeed file if opened in the notepad (or any text editor should look like following).

Line 1:  "Main Category","Sub Category","Mirco Category",Category ID,"Product Name","Product Description",Product Image,Price,"Merchant's Comments","Brand",DetailURL,BuyURL,"Shipping Info","Product Code",Availability,USER_PRODUCT_ID

Line 2:  "Men","Dress","Cap Toe Shoe",,"Como - Geniune Leather Shoe","With great styling and versatility, these Italian made dress casual shoes are styled right for today's more relaxed business or casualoccasions. Padded glove leather linings provide pillow-soft comfort, while full-length leather covered PU footbed help reduce step shock and conform to the foot for a customized fit. Equipped with our exclusive First/Flex advantage, the Como is sure to provide wearable comfort and style for a long time to come. Available Colors: Black, Brown.",http://www.shoebuy.com/pi/bosto/bosto7680_lg.jpg,79.99,"merchant's comments here","Bostonian",http://www.shoebuy.com/sb/s.jsp?Style=7680&Substyle=7680,http://www.shoebuy.com/sb/s.jsp?Style=7680&Substyle=7680,"Free Shipping in US","MC-003",Yes,

Line 3:  "Men","Dress","Oxford",,"Bari ","With great styling and versatility, these Italian made dress casual shoes are styled right for today's more relaxed business or casual occasions. Padded glove leather linings provide pillow-soft comfort, while full-length leather covered PU footbeds help reduce step shock and conform to the foot for a customized fit. Equipped with our exclusive First/Flex advantage, the Bari is sure to provide wearable comfort and style for a long time to come. Available Colors: Black, Brown.",http://www.shoebuy.com/pi/bosto/bosto7682_lg.jpg,129.95,"merchant's comments here","Bostonian",http://www.shoebuy.com/sb/s.jsp?Style=7682&Substyle=7682,http://www.shoebuy.com/sb/s.jsp?Style=7682&Substyle=7682,"Free Shipping in US","MC-004",Yes,

Line 4:  "Men","Dress","Oxford",,"Pavia","With great styling and versatility, these Italian-made dress casual shoes are styled right for today's more relaxed business or casual occasions. Padded glove leather linings provide pillow-soft comfort, while full-length leather covered PU footbeds help reduce step shock and conform to the foot for a customized fit. Equipped with our exclusive First/Flex advantage, the Pavia will provide wearable comfort and style for a long time to come. Available Colors: Black, Brown.",http://www.shoebuy.com/pi/bosto/bosto7683_lg.jpg,96.95,"merchant's comments here","Bostonian",http://www.shoebuy.com/sb/s.jsp?Style=7683&Substyle=7683,http://www.shoebuy.com/sb/s.jsp?Style=7683&Substyle=7683,"Free Shipping in US","MC-005",Yes,

If you open the same CSV in the Microsoft Excel, it will look like following.

Please note that column numbers corresponding to the category, product, prices (and other information) are very important and they should be noted carefully. This information will be required inorder to import a feed file correctly.

In the above feed file, first column is for main category so it is column number ZERO (Start your count from ZERO). Second column is for sub category so it is column number ONE. In this manner you can calculate the column number for each piece of information.

Now go to the admin section of your script. Goto Manage Merchants » Export/Import Products » Import - FEED
Select a merchant (for whom you will be entering products and updating prices) and provide a datafeed file. Click on Upload button. This will upload the datafeed from your computer to the server (Make sure you have given write permissions to the sub-folders under upload directory).

Once the file is successfully uploaded a new page will appear giving you information about the lines in the uploaded file and also asking you about the delimiter and column numbers corresponding to the categories, products, prices (and so on) in the uploaded file. Specify the delimiter and also specify which column is for which information (Remember: You have noted this information in the begining.). Once you have specified this information hit the continue button.

After pressing Continue button a new popup will appear (Make sure your popup blocker does not block this popup) which will update you on the progress of the import procedure. When this process is done a summary page will display a brief report of the process. This page will also include a link to a detailed LOG File which was created while the import procedure was running.
This LOG file can help you in understanding what actions where performed by the import process.

That's it you have successfully imported your datafeed. You can repeat this process for a different datafeed.