The classic implacable advance, the "Steamroller" approach uses serried ranks of infantry backed up by slow moving fire support to crush the enemy. This option is most often used by armies such as Necrons or footslogging Orks, though I've seen Space Marine armies use it as well. Overall it can be an effective if somewhat boring manuever, but your army needs to be very resilient and/or very numerous.

The idea is to march forward, staying in formation, and laying down a lot of firepower to weaken the opponent. Once in range, any assault troops will finish off the survivors. A battle where both forces use the "Steamroller" tactic is a classic example of attrition warfare.
 

Usually employed by static firepower armies, like the Imperial Guard, the "Gunline" is a very one-dimensional tactic. The basic premise is to line up your army into a firing squad and blast your opponent into ashes.

If the table is fairly open, you can usually lay down enough firepower to cripple the enemy. However, if there is suitable terrain to hide behind, you can be fairly certain your opponent will attempt to use it to try and get to you. Any such approaches to your line should be covered by indirect fire weapons or rapid response units (preferably both). If an opponent gets into the battle line of a static firepower army, it's usually bad news.

The "Gunline" is also a horrible tactic to employ if the mission requires you to move and capture territory or objectives.
 

The "Swarm" is an assault oriented tactic that is mostly the domain of the Tyranid, although any army with fast moving troops can attempt it. The general idea is to attack in waves.

The first wave consists of fast moving troops. These poor schmucks will have the thankless duty of getting shot to bits. Any that do manage to get through will then try to tie up enemy units in close combat until the slower second wave can get there and help mop up.

I've seen this tactic used with mixed results. It either wins big or loses big, often depending on available cover in the middle of the board, and the efficiency of enemy shooting.
 

The concept behind this tactic is to deploy a tight group of hard units along a short part of your battle line, backed up by hard hitting infantry. Dreadnoughts or tanks are typical for this tactic, though monstrous creatures would work well too.

This "Spearhead" will then move forward, blasting away at the enemy while providing cover for the units behind. Once the spearhead punches a hole in the enemy forces, your infantry can roll in behind and start mopping up what's left of their battle line.

This tactic works best if the troops behind the spearhead are close combat oriented. You will be in close proximity to the enemy when you engage, so ranged firepower will be of limited use. I've used the spearhead myself and you'll generally get only one round of shooting from them before you're in the enemy's face.