Hardware

Ecigs heat liquid to create vapour in the same way steam is made when boiling a kettle.

The liquid is soaked into the filler of a cartridge or dripped into the atomiser where a coil is heated with power from the battery after the switch is activated.

The components of an ecig are - cartridge/mouthpiece - atomiser - switch and power.

Picture from Exogenesis

Power is activated with either an automatic or manual switch. Automatic switches work when you draw through the device and manual ones when you press a button.

Cartridges and atomisers may be combined in some versions to form a single 'cartomiser'. Cartridges are usually refillable, cartomisers may not be. Cartridges and cartomisers are commercially available in nicotine strengths from zero to 36mg per ml (3.6%). 15-25mg satisfies cravings for most people.

Atomisers have limited interchangeability, popular size connections are 510, 801 and 901 (in practice RN4075 atomisers are more often used with this connection type and are mistakenly called 901s). KR808D-1 cartomisers are also popular. Adapters are available to convert thread sizes of otherwise incompatable atomiser and battery combinations.

New vapers tend to want hardware that looks familiar so that it's less of an upheaval to swap from smoking, the RN4081 supermini is a common cigarette lookalike. Once familiarised with vaping, people tend to prefer devices that won't be confused with cigarettes and which work better, that means branching out to different shapes, sizes and colours. The small batteries standard with kits are usually the first piece of hardware to become obsolete and enthusiasts can often be found using a USB passthrough or a battery case (mod) that looks nothing like a cigarette. Bigger batteries mean more power for longer but the trade off is weight, unconventional looks and initial investment costs.

The Joye eGo and variations - Riva and Tornado, are popular mass produced devices at the moment.

Impartial ratings are very hard to get because there is a lot of scamming and deceptive undercover marketing of ecigs and liquids. For advice about hardware, liquids and techniques one of the best things to do is to visit an independent forum where enthusiasts are usually very happy to help. Healthy skepticism when getting information goes a long way to getting products you will be happy with.

Higher cost is not necessarily a reflection of higher quality, paying for branding and advertising won't give you a better product and most of them are from the same few factories anyway so try to shop around and find the best deals. A trader with good customer care and aftersales service is worth their weight in gold for new users.

You should be able to get started for less than US$70 unless you live somewhere where trade is difficult. A minimum starter kit would be:
2x batteries
1 atomiser with 5x cartridges or 5x cartomisers
a charger
10ml of eliquid