The first great expansions to one of the first great first person shooters in history.
Mission packs, as they were called in the 90's, where expansions to the core game, usually being slightly diverse in order to create more unique gameplay, creating something new and fun. Opposing Force was famous for this, as being the best expansion ever, to the best game ever.
The expansion introduced so much more to gamers that it almost felt like a completely different game altogether.
In this expansion, you are still given Easy, Normal, Hard mode, and even the Training Room (which is completely different from the first). There is even multiplayer mode that introduces many new weapons and game modes, even a capture the flag based set of maps (which are arguably the most trade markable multiplayer element the game introduced to Half-Life).
Other features were new friendly and unfriendly NPCs, new weapons (both organic and non-organic), new voice actors (some are kinda bad compared to today's standard), new models, and of course new maps/areas of Black Mesa.
Since I never release too much back story in my reviews, I'll just say: You're character is Corporal Shepard, a soldier who has just gotten out of Boot Camp and is now on his first assignment, which is located at the Black Mesa research facility. Him and his squad do not even know where they are exactly going, and barely know why they are being sent in. Just as they are coming in for a drop from one of the helicopters, one of the choppers gets shot down from they sky from an unidentified flying object (yeah, sorta like a UFO), and they all eventually crash and get in the heat of a battle with alien looking creatures. You become knocked out and later wake up inside the research facility in one of the labs, along side other soldiers who are dead.
The game is very long compared to the later released Half-Life: Blue Shift, giving about 10-20 hours of gameplay through the first run. There are still Easy-Medium-Hard difficulties and you even get a new multiplayer mode. There is a new hand mode for your character and many new weapons (you strangely even wield a Barnacle at one point in the game).
It's has everything that made Half-Life so fun. A great story mode with an excellent multiplayer as a bonus. An easy earned 10 from me. Play it, you'll get what I mean.