The yoga cards you choose will largely depend on your existing knowledge of yoga, and how you want to use them. If you simply want to use the cards to jog your memory while you're doing yoga, or to visually plan the sequence of asanas you will follow, then basic visual yoga cards will probably be all you need. If you are still working on learning the postures, however, or you need more detailed information about potential modifications, ways to regulate the breath, or the length of time to hold the posture, you might want to choose more advanced cards with written instructions on the back.
For the intermediate to advanced yoga practitioner who wants an easy way to plan a yoga workout ahead of time, getting a set of yoga cards can be a great way to do this. This allows you to visually lay out the cards in front of you and simply follow the postures. These can be purchased online, or many websites allow you to simply print a series of cards, and then cut them out. They do not provide any direction on sequencing or modifying the poses, but for people who have already studied yoga, a simple visual cue may be all that is needed.
If you are more of a beginner, you will want to select a deck of yoga cards that provide more instruction. Many of them are available for different levels of ability or a different intended purpose, such as yoga for relaxation or yoga for weight loss. The benefit to choosing these decks is that not only do they provide additional information on each pose, but they also generally provide information for various sequences of poses to allow them to flow together. The cards may be numbered or color coded to make this easier.
In addition, it is important for beginners to have enough instruction when starting yoga. It might be a good idea to take a few classes with an instructor before moving on to the yoga cards. Then, the descriptions for modifying and deepening the poses on the back of the cards will make more sense. Also, rather than choosing cards that just show one form of the posture, choose cards with a number of smaller pictures that show how the body moves in the different positions throughout the pose. This will be helpful for making small corrections.