Some of the principles of Islamic banking are similar to more general ethical investment. Others are related to avoiding the use of interest payments and speculation.
Banking without the payment of interest creates complexities and requires a range of products designed with this ban in mind. The ban on interest is linked to the disapproval of usury. This is something Islam shares with many other religions, including Christianity and Judaism. However the strict Islamic prohibition is stronger, in that it bans all interest, not just usurious (exploitative or unfair) interest rates.
Islamic banks replace interest payments with profit shares. Depositors get a share of the profits earned by their deposits.
The profits come from hire-purchase and leasing, sale and buy-back arrangements, service charges, purchases on behalf of a customer who later pays the bank back with a agreed mark-up.
There are other restrictions, very similar to those in any type of ethical investing. This means not investing in non-Islamic businesses such as alcohol, pork, non-Islamic banking etc.
As well as specialist Islamic banks, many major banks have Islamic banking operations.