What's the difference between Search Engine Marketing (SEM) and Search Engine Optimization (SEO)?

SEM it is when you buy the adspace on the first page of the SERP for the certain keywords
SEO iot is when you reach the first page through on site and offesite optimization. Could be done as by yourself as well as could by kind of the paid service

SEM is commonly reffered to paid listing of search engines while SEO is stand for organic listing.

SEM/search engine marketing includes SEO, PPC/paid search, social bookmarking etc.

So SEO is just a small part of SEM.

Both are the big concept itself and have difference also,search e- book on Google.

What are those major differences? I was kind of expecting someone explaining them in more detail than just saying "they have differences".

OK, I will clarify it for you

I hope you trust wikipedia

SEO - SEARCH ENGINE OPTIMZATION - http://en.wikipedia.org/wiki/Search_engine_optimization

SEM - SEARC ENGINE MARKETING - http://en.wikipedia.org/wiki/Search_engine_marketing

If you have more questions, just ask

That is clear.
I assume OP is intersted which one is more profitable?

SEO is old and outdated. All you have to do is write clean code, design the site so it's user friendly, and write original content. Beware of anyone trying to sell you SEO services.

How do you figure?

SEO techniques should be obvious to any professional web developer (ie. title tags, proper elements, semantics, keywords, etc).

It's important to make websites and content for humans, not for search engines.

If SEO was so obvious, there wouldn't be million of topics written to explain it. And Google has always made it clear that their recommendation is to be relevant to the query.

There are a million topics written on anything and everything, including brushing your teeth and how to iron or tie a knot. Doesn't mean they're not obvious.

Well, I'm going to have to agree to disagree with you. There are some obvious things about SEO, but little about SEO is old and outdated. I can think of a lot of confusion about the significance of PageRank - where Google is sending mixed signals. On one hand they're saying it doesn't matter anymore and on the other, they just updated their tables.

Let's see "brushing your teeth" = 355,000 results
"how to iron" = 8,600,000 results
"tie a knot" = 1,960,000 results
"SEO" = 214,000,000 results
And I could be mistaken, but I'm not seeing entire forums dedicated to "brushing your teeth"

I wish it was as easy as just creating content for websites and humans, but the stakes are too high when it comes to generating a revenue base online, or branding your name.

I totally agree with you on those point, however as you and I know not everyone else does, hence the need for SEO, SEO ready blogs, design and eCommerce are the norm now, what about people who missed the bus and didn't SEO from start.