Honduran money is actually pretty easy, because the exchange rate is close to 20 to the US$. The current rate is actually almost 22 (when I first came to Honduras in 1998, I think it was about 11 lemp to the dollar, and kept falling until it was held at 18.9 for several years, and then started dropping again when it was allowed to fluctuate), but by thinking of it at 20, it's an easier calculation. Just slide the decimal one place, and divide by 2 to get a rough approximation of the US$ value. The currency is called lempira, or 'lemp' for short. 50 lemp? Slide the decimal, 5, divide by 2 is 2.5, so 50 lemp is about $2.50
Honduran bills come in the following denominations: 1, 2, 5, 10, 20, 100 and 500, so the largest bill is a value of about $25. 500 lemp bills are very difficult to break: most small shops and taxi drivers don't have change on them for this, so make sure you keep smaller change handy for transportation.
Coins in Honduras do exist, but almost no place uses them (when you consider that the smallest bill, the 1 lemp note, is less than 5 cents, there's no items that cost less than that, and grocery stores usually just round the figure). If you like to collect coins, the banks have them. Coins are called centavos, and there are 5 centavo, 10 centavo, 20 centavo and 50 centavo coins.
On Roatan, US dollars are widely accepted, so as long as you have small US$ bills, it's not necessary to get them exchanged. When paying in US, sometimes your change will be given in US, but often will be given in lempira. Banks are very picky about the acceptable condition of US bills, and therefore all stores/businesses are picky too. Bills cannot have any small tears in them or any small piece ripped off, and sometimes won't be accepted if they've been written on.
ATMS on the island mainly dispense in lempira, but there are also several ATMs (including the one at the airport and the one by Coconut Tree in West End) that also have the option of withdrawing in US$, which is handy if you'll be traveling on to another country and want to make sure you have some US currency. Land border crossings have money changers to exchange lempiras to whatever the neighbouring country's currency is, as do airports if you're taking a flight to your next country, but many times at the banks themselves in neighbouring countries, lempiras cannot be exchanged.