Most would-be travellers to Cuba ought to have heard of Cuba's dual economy by now. If not, I suggest reading up on it - there is a lot of interesting literature on the subject. And in a future post I will publish my two cents' worth on why I think it ultimately will destroy the equality that Cuba strives for. :-)
This article focuses on practicalities, so practicalities are what I will talk about for now. The Cuban Convertible Peso (CUC) is the currency that tourists use. This is approximately one US dollar, although it is in no way pegged to the dollar. Convertibles (as they are commonly called) cannot be purchased outside Cuba. This means you need to bring in foreign currency and buy your Convertibles when you get to Cuba.
As with most countries, you should carry your travel money in a mix of cash and credit cards/travellers cheques.
Most international currency is readily accepted at change booths (Casas de Cambio, or CADECAs), including Sterling Pounds, Euros, Canadian Dollars and Yen. American Dollars, while also readily accepted, are penalised by a further 10% "tax" when converting. So, given a choice, don't take US dollars. Regarding rates, all CADECAs and official establishments that change money such as hotels have the same fixed rate since this is state controlled. You can save yourself a lot of hassle with this knowledge - there is no bargain hunting to be done looking for the best conversion rates.
Also, make sure you ask for your change in small notes when you convert money. Most people don't have much money, and as a result not that much change (the number of times I've heard No hay cambio! ... ). They aren't used to dealing in big bills either. If at all possible, try not to hang on to anything larger than a 20 Convertible bill. Stick with the 3, 5 and 10 Convertible ones.
US credit cards will not work in Cuba. This includes all American Express cards, as well as Visa and MasterCards issued by US banks (e.g., a CitiBank Visa). Visa and MasterCard issued by non-US banks work fine, however (I used an HSBC Visa card).
Most establishments in Cuba don't accept credit cards, with the exception of a few major hotels in Havana, and those that do will charge an 11% commission. Credit cards are still handy for cash advances though. CADECAs will give you a cash advance on your credit card (still charging the 11% commission) and ATMs, or cajeros automáticos, as they are called there (yes, even the ATMs will slap on the 11% charge) accept these, and it is often safer than carrying cash.
In terms of exchange rates, I found that, despite typical Visa cash advance fees and the 11% commission, I got (slightly) better rates using an ATM rather than changing Sterling bills - the ATMs did some wacky conversions to US Dollars first and then back to Pounds. Not sure how this would work with other currencies.
Anyway, carrying one or two Visa cards were very reliable as a backup and it means you don't have to carry all of your holiday money in cash. Keep in mind that not all towns have ATMs. Viñales, for example, doesn't, despite being a popular tourist destination. Havana, Cienfuegos, and Trinidad all do though.
Just don't bother. Establishments don't know what to do with these, and neither do ATMs.
I didn't use any, but I've heard that American Express travellers cheques are worthless in Cuba. If you do take travellers cheques, make sure you take Thomas Cook, Visa or MasterCard travellers cheques. I found it very hard to get my hands on these in the UK though as most places that sold travellers cheques only had American Express. Make sure you check before you buy any.
As for Cuba's other currency, the Cuban Peso (CUP), also called monedas nacionales and abbreviated to MN, is pegged at a rate of 25 CUP to 1 CUC. You can get your hands on these easily enough at most CADECAs, although their use is limited to getting street food, ice creams and fruit at markets. For most other things - restaurants and cafés, transport and accommodation, beer and rum - you need the real money: Convertibles.
Be careful of the currency though, they can look similar (especially the coins) and it is not uncommon for unscrupulous bar staff, taking you to be too drunk, naíve or both, to give you your change in CUP instead of CUC. Make sure you check your change.
WebHavana.com has pictures of notes and coins of both currencies, so you can differentiate.