Alongside is listed as a synonym as 'beside', but there is a slight difference. 'Alongside' gives a picture of being closer, touching or almost touching. Beside would give a picture of being near, but not touching or not extremely close. A man is standing with his wife beside him. I would think they are separated by 20 cm. If they are closer and holding hands, I would tend to say that she is alongside him. Note that I could have interchanged the words in the above two sentences and still be correct. The difference in use is a tendency, not a hard rule.