What it says on the tin: simply a pencil sketch with the option for adding detail. A basic sketch, completed in mechanical pencil to specifications, can be added to with more complex details but the more being added, the more likely it'll go up the scale in terms of price.
As with pencil sketches, these are the inked versions. The level of detail can be adjusted accordingly. This option includes also hyper-detailed sketches in pencil only that stray out of the price range of the previous option. Level of detail can vary. For example, an inked basic would be more like: