Abroad /əˈbrɔ:d/
adv. 到国外，在海外;广为流传地
It means that sb. goes to a foreign country, which is separated from the country where he/she lives by an ocean or a sea; or refers to something which is talked about or felt by many people.
它指的是某人去一个与自己居住国隔海相望的其他国家;或是指被很多人谈论或有所感受的事物。
They decided to spend their honeymoon abroad.
他们决定去国外度蜜月。
There was news abroad that a change was coming.
盛传即将有一个变动。
Aboard /əˈbɔ:d/
prep. 上车;在(船、飞机、车)上，上(船、飞机、车)
adv. 在船(或飞机、车)上，上船(或飞机、车)
It means on or onto a ship, plane, bus or train.
它指的是在(船、飞机、公共汽车、火车等)上;或是上了(船、飞机、公共汽车、火车等)。
Tom was already aboard the plane.
Tom已经登机了。
It had taken an hour to load all the people aboard the ship.
花了一个小时才让所有乘客都上了船。
最后总结一下，
这两个词虽然长得很相似，
但意思截然不同。
“abroad”是副词，
主要意思有两个，
“到国外，在海外;广为流传地”;
而“aboard”则为介词或副词，
意思是“在(船、飞机、公共汽车、火车等)上;
或是上了(船、飞机、公共汽车、火车等)”
