Slav & Semi Slav

What are White's objectives and plan against either?

If black adopts a Semi Slav then white's objective should be to prevent the e-pawn from advancing, which blocks the c8 bishop.

woo! Semi-Slav!

They are different systems, and White has many approaches.  This is a broad question.  As thechessvids explained, once Black commits to an early ...e6, then the c8-bishop is locked in the pawn chain temporarily, while if Black waits, then he/she has the option for Bf5, for instance.  

I think it might be best at first to adopt a good, solid opening setup, such as an early e3 for White, in case of dxc4, then Bxc4 with a simple opening scheme.  Otherwise, you might be getting into more complex lines, such as the early a4 lines.  

My advice, if you don't know the lines, is to develop simply...d4/c4/Nc3/e3/Nf3/Bd3 etc...this will get your pieces off the back row, and if you want, you can even take the pawn on d5 with cxd5 and prepare e4 with f3...

Come to think of it...if you really have your heart set on this move order, it might be advisable to play 3. e3 before you commit either knight, - this will eliminate all the funny dxc4/b5 pawn chains that are famous in the Semi-Slav.

Make black pay a price for capturing and holding your c4 pawn.

Thanks Bird. That's good advice. I'd really like to find a line where White gets an advantage even though "normal" moves are made by Black. 

Ah yes! I was really worried about my spelling. As a matter of fact, I lay awake at night worrying about it.