Logos can be created and supplied in many different formats. Before having a logo created you should be aware of this and discuss with your designer to ensure that you have the correct formats.
A .jpg file extensions is essentially an image that is saved as a flat graphic and will allow you to use it on leaflets, business cards and your website but if you are wanting to be able to use the logo on a large scale such as an advertising board or banner then you will need it as a Vector or .eps file.
A vector file will turn the image in to points and is kind of like a set of instructions e.g point A to B is 10 pixels wide at 300 degrees angle. Therefore if you wish to increase the size by ten it will keep the same angle but multiple the width by ten and length by ten accordingly to ensure the image is not distorted.
Logo prices can vary from £30 up to the thousands so be aware that for a £30 logo you may only be getting a .jpg version.