SCRAM is a modification to the original Half-Life single player game. It does not have any new content such as enemies, weapons, or maps.
The intent is two-fold:
1) Make HL more challenging, Enemies are tougher, faster, and smarter. Although your weapons are just as strong as in normal HL, you can’t carry as much ammo for most weapons, particularly on the higher difficulty levels.
2) To offset some of the increased challenge, the real heart of SCRAM is a reward system. In each map (after the tram ride) there are 3 lambda nuclei that can be found. Finding the nuclei can earn you rewards (ammo, health, armor) at the very end of each map. Find one and you might be rewarded – find two or all three and you’ll definitely get a nice gift at the end of the level.