Basics: Blocking 1

So,

We begin our look at basic tactics for 40k with blocking. This is the act of placing your units in a position where they effectively prevent an enemy unit(s) from accomplishing their objective. In this example, Khorne Beserkers are rushing towards some Devastators in order to chop them up. Fortunately the Devs have some mates nearby who can help, letting them continue to rain firepower down on other targets that are more important. Here's the scenario, let's call it turn 2:

So as we can see, the Zerkers basically have a clear run to the Devs, and the Devs are never going to kill all of them before they hit, never mind the fact that they should be shooting tanks instead. Here's how the Raven Guard player deals with them with his turn 3 movement.

The Rhino has moved 12" in order to block the Zerkers. The Zerkers have a choice of either ignoring the Rhino and moving through the difficult terrain, slowing them down, or of assaulting the Rhino and trying to destroy it.

The Zerkers have chosen to try the difficult terrain in their turn 3, but roll badly, and end up not moving very far. The Zerker player then chooses to assault the Rhino in order to garner some extra forward movement.

The 5 marines in the Rhino pop out when it becomes wrecked (something on the order of 40 S5 attacks will do that, even needing 6s to hit) and pass their pinning check.

On turn 4, they move out into a line and shoot the Zerkers a bit, but don't kill anything. They do not assault, because the odds are reasonable that they could lose and run/die in a single round of combat, freeing the Zerkers up to assault the Devs turn 4. On the Zerker turn 4, they assault the Marines, because otherwise they have very little choice in terms of how they are going to reach the Devs.

At the start of turn 5, the Zerkers have wiped out the Marines and consolidated forwards at the end of their turn 4. The Devs have the choice of either firing their heavy weapons one last time at something elsewhere on the table, shooting their heavy weapons at the Zerkers and get charged, or pistol the Zerkers and charge themselves, hoping to not die in one round of combat. Each choice has it's pros and cons, and depends on whether there are any strategically important targets in line of sight, and what the assaulting unit is. If it was Guardsmen, pistols and charge would probably be more effective than 4 lascannon shots, if it was terminators then the lascannons would probably help more. And this is the final result:

The Devs decided to pistol and charge, because this way they deny the Zerkers Furious Charge, and the +1A for assaulting, which means they have a decent chance of locking them up for a turn or so.

Space Marine Gains:

Turn 3 shooting with 4 heavy weapons
Turn 4 shooting with 4 heavy weapons
Tying up an elite combat unit for two full game turns, if not 3

Space Marine Losses:

 Two kill points

As the Space Marine player, it is your job to weigh up the pros and the cons of blocking, and decide whether it is worth sacrificing your own units in order to simply delay an enemy unit. This will differ from unit to unit, for example versus 10 Paladins, the answer will be "Hell yes" but versus 10 Ork Boyz the answer may well be "Hell no". One of the important skills of being a general is what to kill, what to avoid, and what to delay.