海藻泛指所有在海里生长的肉眼可见大小多细胞藻类生物。有时，亦包括一些其他近似，但不同种属的生物。

藻类涵盖了原核生物、原生生物界和植物界。原核生物界中的藻类有蓝绿藻和一些生活在无机动物中的原核绿藻。属于原生生物界中的藻类有裸藻门、甲藻门（或称涡鞭毛藻）、隐藻门、金黄藻门（包括硅藻等浮游藻）、红藻门、绿藻门和褐藻门。而生殖构造复杂的轮藻门则属于植物界。属于大型藻者一般仅有红藻门、绿藻门和褐藻门等为大型肉眼可显而易见之固着性藻类。此类大型藻几乎99%以上之种类栖息于海水环境中，故大型藻多以海藻称之。另外，有些肉眼可见的固着性蓝绿藻和少数之硅藻严格而言应该亦属于大型藻的范围。

Fucales is an order in the brown algae (class Phaeophyceae). Members of this order are fucoids. The list of families (see box at right) in Fucales, as well as additional taxonomic information on algae, is publicly accessible at Algaebase.

The Class Phaeophyceae is included within the Division Heterokontophyta. This name comes from the Greek word phaios meaning "brown" and phyton meaning plant. They include some of the largest plants in the sea, some however are small and fine in structure.