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Essentially it means from one point to another. You define the points.

The most common situations you'll find this word is:"Across a bridge", "Across town", "Sitting Across from you".

Across a bridge means: from one end of the bridge to the other. Across town means the same, from one end of town to the other. We typically use it to comment on how far the distance is. Example: "I went all the way across town to buy some bagels". "Sitting Across from you" is when some one is on the other side of you. Imagine you're sitting at a table. Some one is sitting next you facing the the same direction as you. The person sitting on the other side of the table facing you is sitting across from you.

Hope that helps.

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Basically, across is used to go from one side to another side via a support, like a bridge over a river or just going past a street. Along is different. Along is used when you are making your path through a reference, for eg, using a map. "He is going along the route given on the map." Since the map is a reference point for where you want to go, Along will be used here.

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It's actually a word with a couple of meanings, but the most common meaning is to be go from one edge of something to another, usually with a connotation of being perpendicular, or covering all the way to another edge. For example a bridge "across" a river goes from one side of the river to the other. On the other hand a path "along" a river goes beside the river.