While trying to identify the plant pictured below I discovered that, all cacti are succulents, but not all succulents are cacti (CactiGuide.com). Cacti has become a term commonly used to describe any prickly-looking desert plant. But there are differences between the two. Cacti (Cactaceae) have small, round structures called areoles from which spines, branches, hairs, leaves and flowers grow. Succulents do not have areoles (ucdavis.edu). Cacti are also only native to the Americas, from Alaska south to Chile. This includes species of cacti that have adapted to survive the freezing winters common to Canada (ucdavis.edu). Succulents on the other hand are native to most parts of the world (ucdavis.edu).
The Century Plant (Agave americana) is a native Texas agave (succulent) and described by some as the largest and most majestic of the native Texas agaves (aggie-horticulture.tamu.edu). Agave americana lives for 10 to 25 years and at the end of its life produces a flower stalk up to 15 feet tall (aggie-horticulture.tamu.edu). It is moderately slow growing, extremely drought tolerant and tolerant of temperatures as low as 20°F (aggie-horticulture.tamu.edu). It is frequently used in landscaping, as it is in the picture above.
Resources
http://aggie-horticulture.tamu.edu/ornamentals/nativeshrubs/agaveamer.htm
http://www.cactiguide.com/cactiornot/
http://ucce.ucdavis.edu/universal/printedprogpageshow.cfm?pagenum=6142&progkey=2080&county=5576