A Logo needs to be created in a format that can be resized without any loss of quality and this is generally done by creating your logo in an EPS (encapsulated post script) format using a specific design program. Quite often the EPS files can then be saved as a PDF file and can be easily opened by Adobe Acrobat Reader.
(This however does not mean you can save a JPG file as a PDF and expect the same result as the file was already a JPG before being saved as PDF).
So why the need for vector based files?
They are incredibly small, scalable and editable images, it is your logos data broken down into geometric shapes, outlines and colour and stored as mathematical equations, creating much tinier files than JPEGS and are easy editable and scalable with the right program.
Other files types your logo may come in such as JPG, PNG etc are pixel based formats that if not sized at the actual size you need the file for can become pixelated, blurry and lose detail.
Therefore in order for a graphic or web designer to manipulate your logo to be used at the correct size for your project it is necessary to give your designer your logo in an EPS format.
So no matter which designer you work with always be sure to get your logo in an EPS format. ;)