There are 4 difficulties in the game available to select on the creation screen when naming your company, these are:
Much easier than any other difficulty; for casual playthrough. You sell 25% more than on medium and start with twice as much cash.
The standard difficulty. Everything is optimized to play well with this difficulty.
Pretty hard compared to medium difficulty, where you start with 20,000$ and sell 83% of what you would on medium.
Very Hard (V. Hard) Edit
The hardest difficulty where you start with no money, so you have to take a loan in the Bank. You sell only 67% of what you would normally because of 0.4 sale multiplier.
Difficulty Comparison Edit
Below is a table containing all values which depend on selected difficulty: