[mary523]

A nucleophile is negative, and it is attracted to something positive.
An electrophile is positive, and it is attracted to something negative.

Every time I look at a reaction diagram, I see electrons moving from areas of high concentration to areas of low concentration.

Sometimes these are "nucleophilic attacks", and sometimes these are "electrophilic attacks". How are you supposed to tell the difference between these types of "attack" when in every case there is just electrons moving from a negative to a positive?

They look the same to me, and honestly I can't tell the difference. How do you know if the nucleophile is attacking the electrophile, or if the electrophile is attacking the nucleophile?

[vansh123]

http://www.masterorganicchemistry.com/2010/04/01/nucleophile-attacks-electrophile/ could guide you a bit

[Babcock_Hall]

I would pick out the central atom if there is one.  The central atom would be the one which is undergoing both the breaking and the making of a chemical bond.  If the incoming atom has a lone pair, then that is a nucleophilic attack.  I think this works well for some reactions, but I can see some potential for ambiguity.

[Vidya]

electrons are always moving from more electron density to less electron density ...so it is always nuceophile and electrophile ...now if the substrate is nucleophile and reagent is electrophile ...then it means electrophile attacks the nucleophile ....if substrate is electrophile and reagent is nucleophile ...then nucleophile attacks the electrophile
Like Alkenes + H2/Pt is the electrophilic addition ...because substrate is rich in electron density and is nucleophile .In this case electrophile attacks the pi electron density 

[clarkstill]

This has always bothered me a little bit, for example when talking about "electrophilic aromatic substitution", the untrained chemist might presume this meant the aromatic system acting as an electrophile, and vice-versa for SnAr. It's a semantic issue I suppose, but I can see why people get confused. Arbitrarily separating your reaction into "substrate" and "reagent" seems a little bit dodgy to me though, from the mechanism's point of view there is no difference between your late-stage alkaloid precursor and the methyl iodide you're using to methylate it. Also, one could imagine a convergent natural product synthesis where the two fragments come together, one as an electrophile and one as a nucleophile...