A randonnée or alpine touring binding's primary function is to hold your boot securely to your ski, but it must also allow the ski tourist to maintain a natural stride while skinning on the flat or uphill. To enable this, all randonnée bindings allow the skier's heel to lift when in touring mode but lock down for descending (telemark bindings also allow this upward movement of the heel, but all of the time). Most have adjustable climbing levels at the heel for varying degrees of slope steepness as well.
There are two basic classes of randonnée bindings - frame bindings, which place a toe and heelpiece on rails with a pivot at or near the toe, and Tech (sometimes referred to generically by the brand name "Dynafit"), which requires a special boot. The Tech system incorporates pins on both sides of the boot toe that hold the toe down but allow the heel to lift, and a separate pair of pins at the rear that engage a slotted plate in the boot heel when skiing. In the pictures below, the top four bindings fall into the first group, while the bottom six are "Tech" or Dynafit-compatible.
Some randonnée bindings require a touring-standard sole shape (higher than an ISO alpine sole at the toe, with a rocker or curvature in the the forefoot), though many will adjust to fit an alpine boot as well. A few (notably Silvretta) have a bale attachment for the toe that allows use of plastic mountaineering boots. To save weight, many of these bindings have no actual toe release mechanism; the bindings release both forward and laterally at the heelpiece. Using your alpine boots with a beefy randonnée binding like the Marker Duke is a popular solution for those just getting into touring, but once you start spending much time away from the lifts you will probably feel an urge to rid yourself of the extra weight (the rigid nature of alpine boots is not ideal for touring, either).
Those coming from an alpine skiing background should be aware that few randonnée bindings (heavy frame bindings like the Duke and Guardian may be exceptions) offer the same level of retention as a good alpine binding. Anti-shock and return-to-center capabilities, in particular, are somewhat limited by the pared-down designs, so expert alpine skiers should exercise restraint and work up to power skiing and air gradually. People new to Dynafit or Tech bindings will encounter a short but fairly steep learning curve - there are a number of critical "tricks" involved in successfully using these bindings, and it helps a lot to read up on their use and/or find an experienced user to help guide you through the initiation.
Back Country Access also markets an adaptor called the Alpine Trekker which allows skiers to use their alpine setups for touring. It is basically a hinging frame that fits into alpine bindings and allows skinning in alpine boots, but is carried in your pack on the way down. The Alpine Trekker, combined with alpine skis, boots and bindings, is a heavy solution and probably not an option if you plan on touring more than occasionally.
For detailed information on most popular randonnée bindings, plus downloadable PDF templates for home-mounting them, I highly advise checking out Lou Dawson's excellent website, http://www.wildsnow.com - his FAQ pages regarding Dynafit bindings are especially worthwhile.
© 2013 Gregory C. Louie